If you have the right attitude, you could start on a BUSA or any liter bike. It's just when you get brave and try things you shouldn't, you will hurt yourself. As long as you drive it civilized, you won't have a problem.
I would get AT LEAST a 600cc bike and even then I would think about something bigger. Just for the fact that if you learn properly and don't wreck in the first 60-days, you will be sick of a 600cc bike after that.
BE SMART, TAKE A MSF COURSE AND BE SAFE!!!