When you start new Android project, with SDK version 23, you will notice e-mail icon, like on picture:

Open activity_main.xml, in my case it is located in app\src\main\res\layout\activity_main.xml, and delete code:

<android.support.design.widget.FloatingActionButton
	android:id="@+id/fab"
	android:layout_width="wrap_content"
	android:layout_height="wrap_content"
	android:layout_gravity="bottom|end"
	android:layout_margin="@dimen/fab_margin"
	android:src="@android:drawable/ic_dialog_email" />

Then in MainActivity.java, in my case it is located in app\src\main\java\com\example\smi\myapplication\MainActivity.java delete code:

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
	@Override
	public void onClick(View view) {
		Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
				.setAction("Action", null).show();
	}
});