how do i open contacts when i click a button defined in main.xml

You can set an Event on Button click by setting an OnClickListener on the Button with the following code, and use Intent to call ContactPicker activity:

button.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                 Intent intent= new Intent(Intent.ACTION_PICK,  ContactsContract.Contacts.CONTENT_URI);

        startActivityForResult(intent, PICK_CONTACT);

            }
        });

and in onActivityResult() process the contact uri to load details of contact.

@Override
public void onActivityResult(int reqCode, int resultCode, Intent data) {
  super.onActivityResult(reqCode, resultCode, data);

  switch (reqCode) {
    case (PICK_CONTACT) :
      if (resultCode == Activity.RESULT_OK) {
        Uri contactData = data.getData();
        Cursor c =  managedQuery(contactData, null, null, null, null);
        if (c.moveToFirst()) {
          String name = c.getString(c.getColumnIndex(ContactsContract.Contacts.DISPLAY_NAME));
          // TODO Fetch other Contact details as you want to use

        }
      }
      break;
  }
}

You should use startActivityForResult

Intent intent = new Intent(Intent.ACTION_PICK, ContactsContract.Contacts.CONTENT_URI);  
startActivityForResult(intent, 1); 

See "get contact info from android contact picker" for more information.