In personal experience I can only speak on Sportbikes, as that is all I have ridden, but, like the others have said, start SMALL (ie a 250 Ninja, or CBR). I started out with a Ninja 250. I now ride a nkna ZX-7R (700 class) on the street, and a Ducati 1098 for the track. I cannot stress enough to GET GEAR! Our sportbike rider's club has a saying, AGATT (ALL GEAR ALL THE TIME). I actually own 2 sets of gear, one set is a nice textile jacket with armor padding, a back protector, Shoei helmet, armored textile overpants, and alpinestars boots+gloves. This is my street gear. Track gear is a full 1 piece leather suit, again with A* Boots/Gloves and an Arai helmet.
If you have ever watched MotoGP or the like (Motorcycle racing) then you have seen the guys who high-side at 100+mph, and they stand up and walk back to the bike... this is partially due to trak design (lots of runoff area) but the MAIN reason is due to GEAR. Include gear as part of the purchase of the bike. if you cannot afford gear and the bike, don't buy it. When (not IF) you go down, it will save your life, skin, etc. Also, if you ever plan to ride a backpack (passenger) get at LEAST a helmet, jacket, and gloves for your passenger. Personally I purchased a Medium helmet, and jacket, and medium gloves, as those tend to be the best "balance" for fitting people. Might be alittle large, or a little too tight, but either is usually tolerable.
After you have the bike and gear, get a riders course! Here in OH Motorcycle Ohio offers the Basic course for 50 bucks. Sign up early, as slots fill quickly. After that, ride, ride ride.. I also HIGHLY recommend participating in at least one track day at a road course near you. Even if you have no interest in going fast, or racing, the track day will do for your handling of the bike at higher speed, what the riders course does for basic awareness and low speed handling. You will learn more in one track day than in years of riding.
With that said, if you have any other questions, I will be glad to answer them for you!