Just include it in your gradle dependencies:
gradle script/ build gradle module

dependencies {
    …
    compile ‘com.mcxiaoke.volley:library:1.0.17’
}

package com.abysamltd.myapplication;
import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

    private Button b_kaydol_screen,b_giris_screen ;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        b_kaydol_screen = (Button) findViewById(R.id.b_kaydol_screen);
        b_giris_screen = (Button) findViewById(R.id.b_giris_screen);

        b_kaydol_screen.setOnClickListener(this);
        b_giris_screen.setOnClickListener(this);

    }
    public void onClick(View v) {

        switch (v.getId()) {
            case R.id.b_kaydol_screen:
            startActivity(new Intent(MainActivity.this, Activity_register.class));
            break;

            case R.id.b_giris_screen:
                startActivity(new Intent(MainActivity.this, Activity_login.class));
                break;

        }

    }

}