Building on what Horstl said, you can think about hobbies and interests that you have and see if that gives you any ideas. For example, my nick (minus the 2 that is only there because I forgot my login with the other one and made a new account here) was chosen as a name for a role playing character in a fantasy game. I liked it, so kept using it for everything else. Some people pick names of characters from video games that they like. Others just use their real name or a version of it. It's all up to you. Pick something that you like and that fits your personality. Just avoid hard to type nicks where people have to keep looking to see how it's spelled or ones with a lot of symbols as those are a pain to type as well.
