In your layout XML:
<TextView
android:id="@+id/myAwesomeTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:text="Escriba el mensaje y luego clickee el canal a ser enviado"
android:textSize="20sp" />
Then, in your activity class:
// globally
TextView myAwesomeTextView = (TextView)findViewById(R.id.myAwesomeTextView);
//in your OnCreate() method
myAwesomeTextView.setText("My Awesome Text");