This smells heavily of homework, so you get no code!
It's pretty easy though:
1) Read the contents of the first TextBox (use it's Text property)
2) Convert it to an integer, using the
int.Parse[
^] method.
3) Do the same for the second TextBox
4) Add the two integers together, and save the result in a third integer.
5) Set the Text property of the first TextBox to the result using the ToString method: result.ToString() will do it.