Crap Advice
Be careful who you listen to on this post. Much like the handgun post, you're bound to get some crappy advice. Maybe its me.
Don't let your first bike be a $5K-$6K brand-new honey of a bike. And you don't need to start on a crotch-rocket. Too much power and too agressive of a riding position gets newbies into trouble. Get a 250cc or 400cc crap-mobile and plan to dump it on its side a few times. Lots of beginners drop their bikes in the garage or in the parking lot. If they get off balance, they can be heavy. No sense in learning on a $20K Harley either.
Once you learn what you're doing (take a Motorcycle Safety Foundation Course or two) and ride for a while, upgrade. If the beater sits in the garage and you don't love riding, at least you won't take a big hit $$-wise. If you love it, sell it and buy your dream machine.
Just one guy's advice.
Nash
p.s. As Kyle noted, these things are dangerous. I'm an ER doctor and the most physically devastated patients are almost invariably motorcycle riders. Usually not at fault. If you take a walk through our Trauma ICU, at least 1/2 of the patients are motorcyclists even though there are not that many riders on the road. If you get hurt, it can be bad. Its not uncommon to have a MC rider in the ICU for MONTHS, in traction and intubated (breathing machine), if they live.