Basically you can just buy the two big coins aka bitcoin and ethereum and just hold them for a long time hoping for big price increases during 2018 like many people do, you can also do the same with alt coins, even though it is more riskier but can be more profitable since some of them 3x-10x in small amounts of time. If you're not interested in holding you can simply day trade bitcoin for alt coins and the other way around. The principle is buying low selling high like you said, but in order to do that successfully you need to learn technical analysis, watch youtube videos/research about it, it's hard to be successful at it though.

Usually people use coinbase to obtain bitcoin with fiat, most likely by bank transfers. So when you wanna sell your crypto you do the same process as you did when you bough just the reverse of it.