Someone knows how I have to do in order to change a "text field" text if a Combo Choice have an exact text, using trigger?

In the "after update" trigger of the"Combo Choice" .. do something like  

if text('Combo Choice') = "Exact Text" then 

    'Text Field' := text('Combo Choice');

end;

**disclaimer.. code not tested.. your mileage may vary.   :)