Whenever you're legal to ride (not sure how old you have to be based on wherever you're from), get a 250 at the biggest.
However, I'd recommend learning your way around the road in a car first. Getting used to all the crazy jackasses on the road is a scary thing if you're learning on a bike. Learn the road with some steel around you, then move to the bike. At least by the time you get on the bike, you won't be as nervous about driving, and will have a better idea of what to look out for, anticipate trouble, etc, etc.