Like many people have already addressed, I'd be too young and have too little influence to do anything at all.
The only thing I could think of off the top of my head is to call in a bomb threat to all of the airports the terrorists took off from, so the planes never took off in the first place.
Unfortunately, who the hell knows right now off the top of their heads which airports those were, and the names of all the terrorists? I know Atta, and that's it, so even if I did know the airports and the times and the flight numbers, I couldn't prove who it was, and they'd just regroup and we'd have 9/24/01.
I mean, if you made the slightest indication you knew anything specific before it happened, you'd get your ass hauled off to the bay.