Change screen Adapter

I have the next part of my program:

  // Si clickem a sobre de qualsevol d'ells...
    lista.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> pariente, View view, int posicion, long id) {
            Lista_entrada elegido = (Lista_entrada) pariente.getItemAtPosition(posicion);

            CharSequence texto = "Seleccionado: " + elegido.get_textoDebajo();
            Toast toast = Toast.makeText(MainActivity.this, texto, Toast.LENGTH_LONG);

I have managed to click on a text to show me a toast, but I can not change activity .. I mean I need to go to another screen for example Pantalla.class.

Can you help me?

Thanks!