Do some puzzles(wargames). For example: http://overthewire.org/wargames/

It will make you search for several special use cases and will give you some experience with the command line. Basically, you SSH into a box, solve a problem and the solution for that problem is the password for the next SSH connection for the next problem.

This one is for beginners: http://overthewire.org/wargames/bandit/