Android - Set text to TextView

I'm currently learning some android for a school project and I can't figure out the way to set text dynamically to a TextView.

Here is my code:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_enviar_mensaje);
    err = (TextView)findViewById(R.id.texto);
    err.setText("Escriba su mensaje y luego seleccione el canal.");
}

This is currently not working and I can't find a way to make it work...

Any help will be much appreciated... Thank you for the time, José.

EDIT: Here is the activity_enviar_mensaje.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    ...
    tools:context=".EnviarMensaje" >
    ...
    <TextView
        android:id="@+id/texto"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/listaVista"
        android:layout_alignParentBottom="true"
        android:layout_alignRight="@+id/listaVista"
        android:text="TextView" />
    ...
</RelativeLayout>

By not working I mean the text shown does not change at any moment...

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");

PUT THIS CODE IN YOUR XML FILE

<TextView
    android:id="@+id/textview1"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>

PUT THIS CODE IN YOUR JAVA FILE

// Declaring components like TextView globally is a good habit

TextView mTextView = (TextView) findViewById(R.id.textview1);

// Put this in OnCreate

mTextView.setText("Welcome to Dynamic TextView");

Well, @+id/listaVista ListView is drawn after @+id/texto and on top of it. So change in ListView from:

android:layout_below="@+id/editText1"

to:

android:layout_above="@+id/texto"

Also, since the list is drawn after textview, I find it dangerous to have android:layout_alignRight="@+id/listaVista" in TextView. So remove it and find another way of aligning.

EDIT Taking a second look at your layout I think this is what you really want to have:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".EnviarMensaje" >

    <TextView
        android:id="@+id/textView1"
        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" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/textView1"
        android:layout_alignRight="@+id/textView1"
        android:layout_below="@+id/textView1"
        android:ems="10"
        android:inputType="text" />

    <TextView
        android:id="@+id/texto"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/listaVista"
        android:layout_alignParentBottom="true"
        android:text="TextView" />

    <ListView
        android:id="@+id/listaVista"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_above="@+id/texto"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/editText1" >
    </ListView>

</RelativeLayout>