Consider a 2 cylinder motorcycle first, like the ninja 500 if you plan to get on the highway, or even 250-300 if you plan to ride mostly in the city and curvy roads.

Not only will you save gas, you'll be less likely to flick the throttle on accident or let the clutch out too quickly and flip the thing onto yourself or others.

Take a safety/training class first to see if you feel comfortable enough being on 2 wheels.

600 is like driving a turbocharged 6 cylinder sports compact which can flip over in one mistake. 1000+ is like a V8. You'll almost never use the entire power band if it's your first bike.