If you haven't ever had a motorcycle before, then get an on/off road dirtbike. And, go ride on the trails. Trails are much more demanding than street driving, in terms of the road conditions, etc. As my friends told me when I first became interested, if you can master the whims of a dirt bike, then you have a fighting chance on the street. And on the street, not only do you have to watch out for yourself and your driving, but everyone elses!
Once you get the hang of a beginner bike (yes, I know, who wants to start small, right? But trust me, learning on a ZX-6 is STUPID...Ive seen many people do that and get hurt really bad.)...then ponder a street bike, like a 600cc or so.
Our local dealership here has a wall of shame. We have 7 CBRs, FZFs, ZX's and such that still have the dealer/temp plate...all bikes bought by beginers and wrecked. Some of the kids were pretty badly hurt, while some just got skinned up. But there you have it, super light, super fast bikes bought by newbies who thuoght they could handle them, and couldnt. Most were dumped on corners, got whacked by an unattentive driver and so on.
So, be smart (and you are by asking)...start with a small bike, learn the basics, then work your way up.