Well for starters you'll need to learn how to ride one from scratch so I'd suggest you attend riding school, Motorbikes aren't as easy to ride as they look. Once you've progressed past your time in Riding school you need to think about what kind of bike you want to ride:
Road Bike:
Pros
- Longer gear ratios, good for Motorway riding
- Extremly nimble, if some goose in a car decides to run you, consider him smoked when the lights change :twisted:
- Wider tyres and bigger brakes, Wide tyres = More grip, Bigger Brakes = Quicker stopping
Cons
- Very expensive to fix if dropped
- Crap turning circle
- Replacement parts can be expensive
Trail Bike:
Pros
- Dirt Cheap to run, over here in Oz: $20 = 250km's
- Take it anywhere, get a set of Hybrid tyres and you can go bush on Sunday and then ride to work on Monday.

- Running costs are minimal
- Tighter turning circle
Cons:
- Short gearing, Dies in the arse after 110km/h :???: (model depending)
- Parts, Not as dear as a Roadie but still enough to hurt your hip pocket
- Some trailbikes don't come with Electric start, How strong are you in the legs?
I'd recommend you get a trailbike to start with so if you drop the bike it won't cost near as much to fix. I'd also suggest you get a bike no smaller than 250cc if you want to keep up with the traffic.
I can not say this enough though: The open road is a
very hostile environment if you're on a bike because you're not even half the size of a car so other motorists will not see you as easily!! Be extremely careful out there, they don't call Motorcyclists 'Organ Donors' for nothing.