what do i have to do when buying a second hand motorcycle?

It's a lot like a car. You buy the bike. You get insurance. You register the bike (you might have to show proof of insurance to transfer the title.) You get a learner's permit and learn to ride.

You can get bike insurance the same place you get your car insurance, or you can go online and try Progressive, GEICO, Esurance, etc. and see what's cheaper.

If you're under 25 insurance is really expensive. If you get an old beater bike to learn to ride on, you might consider getting only liability insurance, which is all that's required by law. It's a calculated risk, you could lose the whole bike, but the thing is most minor accidents are going to cost less than your deductible. It doesn't make a lot of sense to pay $2500/yr to insure a bike worth $3000.

If you're in the US, I'd say the MSF course (Motorcycle Safety Foundation) is a good idea. As dangerous as it is to ride a motorcycle, it's about twice as dangerous to LEARN to ride, so a safety course is not a bad idea. They don't teach you to ride, they teach you some important safety habits to keep you safe while you learn on your own--which I think is the only way.

You should ride slowly around a quiet residential neighborhood or quiet country lanes (avoid traffic) until all the controls are 'automatic', that is, you can clutch and shift fairly quickly and you don't have to think for a second to remember where the brakes are. Then venture out of the neighborhood into moderate traffic. Just remember cars are the biggest hazard. Drivers of cars don't always see you so it's up to you to stay out of their way.

And please don't get a hot sportbike to learn on--CBR, GSX-R, R6, Ninja ZX, etc. They're great bikes, marvels of modern engineering, but they're not easy to ride, not safe for beginners. Almost anything else between 250 and 650cc is fine. And if it's a cruiser you could go bigger because cruisers have a lower center of gravity so they're easier to manage for their size.