I would learn how to ride on something other than a 600. Take a riding course, learn on a dirtbike, get a ninja 250 like skelly said above. They are super cheap, and cheap to insure. Also, make sure you have good health insurance...motorcycle insurance doesn't cover any injuries on you. I wouldn't go anything higher than a 600, and even that is pushing it IMO for someone who has never ridden before. But if you must, sit on a few and find what is comfortable for you. I wouldn't be trying to go fast or hit any backroads for a long time...and don't go cheap on gear. Get a good jacket, and a good helmet..ie Shoei/Arai.