I'm not particularly interested in changing any particular event in my life (I've had some shitty moments, but nothing I'd classify as "must change!"). I'd probably do what I already did, but with far more sex and far less smoking. Maybe not be so late to chat up certain girls whilst in High School. The biggest thing I think I'd change is College, where I would get a different degree (maybe something in Business or Teaching). Maybe work a bit harder to keep my High School weight...
And I'd DEFINATELY start trading in stocks. And I'd probably call in an anonymous "threat" to the WTC an hour or so before the first plane hit, try to at least lessen the casualties. I'd do more, but chances are there'd be a Cassandra type effect, 9/11 would still occur, and I'd get lumped in as "The Mastermind Behind 9/11" or something.
Of course, if this were a Groundhog's Day effect (where I keep living from 16 to 28 all over again), I would educate myself to the point of achieving my maximum potential, and then harness the power of this temporal paradox to acheive apotheosis.