Radio Buttons

I have a group box with two radio buttons on it, button 'a' and button
'b'. When you first start the app neither button is clicked. If you
click on anywhere on the group box, button 'a' will fire. If you click
on button 'b' it will fire and when you click on the group box nothing
happens (this is the desired result). I looked in the help for radio
buttons and it says that there is usually a default button. I want to
make it where the user has to click the button and if they click on
anywhere on the group box nothing will happen.