To post something you need to create an activity from where you can send data to server however you can do the same from
your fragment but if you want to show posts in fragment, you have to do so.
Here I have used Volley Post Request so you should add Volley Libraray 


    implementation 'com.android.support:design:28.0.0'
    implementation 'com.android.support:support-v4:28.0.0'
    implementation 'com.android.support:cardview-v7:28.0.0'
    implementation 'cc.cloudist.acplibrary:library:1.2.1'
    implementation 'com.android.volley:volley:1.1.1'
    implementation 'de.hdodenhof:circleimageview:3.0.0'
    implementation 'com.theartofdev.edmodo:android-image-cropper:2.7.0'
    implementation 'com.android.support:recyclerview-v7:28.0.0'
    implementation 'com.mindorks.android:prdownloader:0.4.0'
    implementation 'com.google.code.gson:gson:2.8.5'


package com.tecent.student_assessment;
import android.content.Context;
import android.content.Intent;
import android.content.SharedPreferences;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.Color;
import android.net.Uri;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import com.android.volley.NetworkResponse;
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;
import com.theartofdev.edmodo.cropper.CropImage;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.util.HashMap;
import java.util.Map;

import cc.cloudist.acplibrary.ACProgressConstant;
import cc.cloudist.acplibrary.ACProgressFlower;

public class CreatePostQueryDoubts extends AppCompatActivity {
    ImageView iv_cancel_post, iv_profile_image, iv_set_image;
    TextView tv_username, tv_btn_post, path_image;
    EditText et_post_text;
    SharedPreferences sharedPreferences;
    ACProgressFlower dialog;
    RequestQueue requestQueue;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_create_post_query);
        iv_cancel_post = findViewById(R.id.iv_cancel_post);
        iv_profile_image = findViewById(R.id.iv_profile_image);
        tv_username = findViewById(R.id.tv_username);
        et_post_text = findViewById(R.id.et_post_text);
        tv_btn_post = findViewById(R.id.tv_btn_post);
        path_image = findViewById(R.id.path_image);
        iv_set_image=findViewById(R.id.iv_set_image);
        requestQueue = Volley.newRequestQueue(this);
        sharedPreferences = this.getSharedPreferences("studentAssessment", Context.MODE_PRIVATE);
        String userdp=sharedPreferences.getString("userdp","");
        requestQueue.add(ExtraFunctions.createImageRequestFromUrl(ExtraFunctions.serverurl+"userdp/"+userdp, iv_profile_image));
        String name = sharedPreferences.getString("name", "");
        String userid=sharedPreferences.getString("userid","");
        tv_username.setText(name);
        //progress dialog
        dialog = new ACProgressFlower.Builder(this)
                .direction(ACProgressConstant.DIRECT_CLOCKWISE)
                .themeColor(Color.WHITE).text("Uploading....")
                .fadeColor(Color.BLACK).build();
        dialog.setCancelable(false);

        iv_cancel_post.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                finish();
            }
        });

        tv_btn_post.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (et_post_text.getText().toString().trim().length() < 5) {
                    Toast.makeText(CreatePostQueryDoubts.this, "Post should contain at least 5 characters", Toast.LENGTH_SHORT).show();
                } else {
                    dialog.show();
                    if (ExtraFunctions.isNetworkStatusAvialable(CreatePostQueryDoubts.this)) {
                        if (iv_set_image.getDrawable() != null)
                            volleyImageRequest();
                        else
                            volleyTestWithoutImage();
                    }
                    else {
                        dialog.dismiss();
                        Toast.makeText(CreatePostQueryDoubts.this, "No internet connection!", Toast.LENGTH_SHORT).show();
                    }
                }
            }
        });


        et_post_text.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
            }
            @Override
            public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
                if (et_post_text.getText().toString().trim().length() >= 5) {
                    tv_btn_post.setBackgroundResource(R.color.green);
                } else if (et_post_text.getText().toString().trim().length() < 5) {
                    tv_btn_post.setBackgroundResource(R.color.smalldarkgrey);
                }
//                Toast.makeText(CreatePostQueryDoubts.this, String.valueOf(et_post_text.getText().toString().trim().length()), Toast.LENGTH_SHORT).show();
            }
            @Override
            public void afterTextChanged(Editable editable) {
            }
        });

    }

    //image picker
    public void openImagePicker(View view) {
        CropImage.activity().setAspectRatio(1, 1)
                .start(this);
    }
    @Override
    public void onActivityResult(int requestCode, int resultCode, Intent data) {
        if (requestCode == CropImage.CROP_IMAGE_ACTIVITY_REQUEST_CODE && data != null) {
            CropImage.ActivityResult result = CropImage.getActivityResult(data);
            if (resultCode == RESULT_OK) {
                Uri resultUri = result.getUri();
                final String path = resultUri.getPath();
                //now we have path of file we should compress image
                iv_set_image.setImageBitmap(compressImage(path));
                String[] paths = path.split("/");
                path_image.setText(paths[paths.length-1]);
            } else if (resultCode == CropImage.CROP_IMAGE_ACTIVITY_RESULT_ERROR_CODE) {
                //on error
                dialog.dismiss();
                Toast.makeText(this, "An error occured! Please try again later.", Toast.LENGTH_SHORT).show();
            }
        }
    }

    public Bitmap compressImage(String path) {
        byte[] imageData = null;
        try {
            final int THUMBNAIL_SIZE = 256;
            FileInputStream fis = new FileInputStream(new File(path));
            Bitmap imageBitmap = BitmapFactory.decodeStream(fis);
            imageBitmap = Bitmap.createScaledBitmap(imageBitmap, THUMBNAIL_SIZE,
                    THUMBNAIL_SIZE, false);
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            imageBitmap.compress(Bitmap.CompressFormat.JPEG, 100, baos);
            imageData = baos.toByteArray();
            Bitmap bmp = BitmapFactory.decodeByteArray(imageData, 0, imageData.length);
            return bmp;
        } catch (Exception ex) {
            return null;
        }
    }

    public void volleyImageRequest(){
        String url = ExtraFunctions.serverurl+"postdoubts.php";
        VolleyMultipartRequest multipartRequest = new VolleyMultipartRequest(Request.Method.POST, url, new Response.Listener<NetworkResponse>() {
            @Override
            public void onResponse(NetworkResponse response) {
                String resultResponse = new String(response.data);
                try {
                    JSONObject result = new JSONObject(resultResponse);
                    String status = result.getString("result");
                    if (status.equals("successful"))
                    {
                        dialog.dismiss();
                        Toast.makeText(CreatePostQueryDoubts.this, "Post uploaded successfully", Toast.LENGTH_SHORT).show();
                        finish();
                    }
                    if (status.equals("error")){
                        dialog.dismiss();
                        Toast.makeText(CreatePostQueryDoubts.this, "Error! Please try again later...", Toast.LENGTH_SHORT).show();
                    }
                } catch (JSONException e) {
                    dialog.dismiss();
                    e.printStackTrace();
                }
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                dialog.dismiss();
                Toast.makeText(CreatePostQueryDoubts.this, "Volley Error", Toast.LENGTH_SHORT).show();
            }
        }) {
            @Override
            protected Map<String, String> getParams() {
                Map<String, String> params = new HashMap<>();
                params.put("userid", sharedPreferences.getString("userid",""));
                params.put("posttext", et_post_text.getText().toString().replace("'","\\'"));
                params.put("email", sharedPreferences.getString("email",""));
                params.put("name", sharedPreferences.getString("name",""));
                return params;
            }

            @Override
            protected Map<String, DataPart> getByteData() {
                Map<String, DataPart> params = new HashMap<>();
                // file name could found file base or direct access from real path
                // for now just get bitmap data from ImageView
                params.put("doubtimage", new DataPart("file_image.jpg", AppHelper.getFileDataFromDrawable(getBaseContext(), iv_set_image.getDrawable()), "image/jpeg"));
                //DataPart second parameter is byte[]
                return params;
            }
        };

        VolleySingleton.getInstance(getBaseContext()).addToRequestQueue(multipartRequest);
    }

    //Google volley
    public void volleyTestWithoutImage() {
        String url = ExtraFunctions.serverurl + "postdoubts1.php";
        StringRequest stringRequest = new StringRequest(Request.Method.POST, url, new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                jsonParser(response);
            }
        }, new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                dialog.dismiss();
                Toast.makeText(CreatePostQueryDoubts.this, error.toString(), Toast.LENGTH_SHORT).show();
//                Toast.makeText(CreatePostQueryDoubts.this, "Error! Please try again later...", Toast.LENGTH_SHORT).show();
            }
        }) {
            protected Map<String, String> getParams() {
                Map<String, String> MyData = new HashMap<String, String>();
                MyData.put("userid", sharedPreferences.getString("userid",""));
                MyData.put("posttext", et_post_text.getText().toString().replace("'","\\'"));
                MyData.put("email", sharedPreferences.getString("email",""));
                MyData.put("name", sharedPreferences.getString("name",""));
                return MyData;
            }
        };
        requestQueue.add(stringRequest);
    }

    public void jsonParser(String jsontext) {
        try {
            JSONObject emp = (new JSONObject(jsontext));
            String result = emp.getString("result");
            if (result.equals("successful")) {
                dialog.dismiss();
                Toast.makeText(CreatePostQueryDoubts.this, "Post uploaded successfully", Toast.LENGTH_SHORT).show();
                finish();
            }
            if (result.equals("error")) {
                dialog.dismiss();
                Toast.makeText(CreatePostQueryDoubts.this, "Error! Please try again later...", Toast.LENGTH_SHORT).show();
            }
        } catch (Exception exception) {
            dialog.dismiss();
            exception.printStackTrace();
        }
    }
}