That'd just require some simple buttons. You'd just put a stop() action in the frame with the choice, then make two buttons. On one you;d put something like:
on(press){
gotoAndPlay("choice1")
}
and on the other:
on(press){
gotoAndPlay("choice2")
}
But that's a very brief description, I suggest taking some tutorials. I find this site very useful:
http://actionscript.org/tutorials.shtml
But there's some good ones all over the place, even right here on Newgrounds (I think there's a tutorials collection or something).