(I'm using my answer from another who asked the same question.)
how large of a person are you, how much riding experience have you had, and are you looking for sport or touring?
If your a relatively small person, and you've never ridden before, a used 250 CC will be a good starter, if your larger, a 500-600 is a good start. DONT GO OVERBOARD ON SIZE OF YOUR FIRST BIKE. its important not to get something that'ts over powered for you, a lot of people make this mistake, and get hurt. Secondly, don't buy new for a first bike, its a waste of money, YOU WILL LAY IT DOWN EVENTUALLY. If someone tells you they've never been down, they haven't been riding long enough. There are two types of riders, who's been down, and who will go down. Not trying to scare you, just my advice. Please get a comfortable helmet, and wear the damn thing.
That being said, now we're on to style, if you're looking for a sport bike, Ninja's are great for what you're looking for, and right in your price range. 250's or 500's, buy cheap, ride it a season or two to get some experience points, and then sell it and move up to a 600-650. (huge difference in 500-650)
If you want a touring, and your small, Honda rebels are a classic, if your larger, suzuki's are nice too.
My advice is my own opinion, take it for what you will, glad to see another getting out and riding. Good luck to you.