Seconded. Do what this man says. UJM or dual sport is the way to start, especially dual sport. I wish I started on one. Height may be an issue for you on that though.
If you absolutely must have a sport bike, get a Ninja 250 and don't pay more than $1500 for it. Year is unimportant. Brand new they MSRP at $3000 and there were factory rebates last year, so don't let anyone scam you.
There's going to be a bunch of naysayer friends of yours who will rag on you for having a small bike, but I'm willing to bet that 9/10ths of them don't have a bike, or let their dad pay for the one they do have. I started at age 19 on a '72 CB500. I paid $400 for it, and fell over on it at least 4 times. Trust me, it will be worth it. Whatever you buy, you'll be able to resell for the same price and then you'll be comfortable moving on to a bigger bike. I promise you that you will thank me if you follow this advice.
--The Fox