Well first of all, congrats on getting your permit!
You don't want to buy too nice a bike to learn on. You're going to be hard on your first bike. You're likely to drop it once or twice, just minor spills or parking-lot tipovers. And if you can't drive a manual transmission car, you're likely to abuse the clutch before you get the hang of it. So you don't want a shiny new one.
A private party will give you a better deal than a dealer. The Kelly Blue Book has two values, one for what the dealer pays for a trade-in, the other for what the dealer charges when he sells the bike. With a private party you split the difference, so it's better for both of you. (Be warned, though, that KBB prices are totally unrealistic. They mean nothing.)
There are a number of web pages with tips for what to look for when buying a used motorcycle, so I won't go into it here. Google "buying a used motorcycle" and you'll see them. A modern Japanese motorcycle is good for years and years, practically bulletproof. Look at replaceable items like tires, chain, sprockets, etc.
The Ninja 250 is a good bike to learn in, in fact it's designed as a trainer, an 'entry level bike'. But if you see yourself riding long distances on the highway you want something bigger. A 250 will go 80 mph (maybe, on a good day) but it's definitely a handful at 60 or 65. A 500 or 650 is not that much bigger or heavier, but you'll be happy for the added stability and stamina.
If you passed the test for your permit, you are probably aware than -learning- to ride a motorcycle is more dangerous than actually -riding- a motorcycle. You probably know already that half of all motorcycle accidents happen to riders with less than 1 year's experience. Not to discourage you, just to show you how important safety is in starting out, how important it is not to take unnecessary chances. And just think, 1 year from now you'll be twice as safe! 8^)
Good luck!
