How do I make sure I do a good job in buying a used car?

I'm looking at a car online right now and I want to buy it from this person. In terms of car functionality, it seems fine. But what I'm wondering about (because I've never owned a car) is that the pictures show the car without a license plate. He mentioned that I have to register the car? How difficult or what problems can arise with that? He did say, however, that it was not salvaged. Also, what other things should I be looking out for? How does this kind of transaction work? Do I pay first or get the car first? Is there paperwork involved in this process? Does registering a car cost money?
(The more information you can give, the better because I am a completely new at this.)
Thank you for your time and help.