Help on making a simple program

I hope this is in the right group:
I’m learning my way through Ruby-Gnome2 right now and am working on a
simple program. All that it is going to do is let the user select a
number from one combo box and another number from a different combo
box and display the sum.

The question(s) I have is a)What widget (I think is the right term) do
I need to use to display the sum? and b)How do I know which signals I
am supposed to use?