Your username (also known as nick) gives one of first impressions of you when introducing yourself online. At the best, your username tells people you're an original person with good taste, and at the worst, it blends you to the mass of people lacking imagination.
GENERAL ISSUES TO THINK ABOUT
Here is a list about issues that are healthy to think about in general. After this section, I'll give you "not to do" and "to do" lists about creating usernames. So...
The most important thing is that YOU like your name. If you don't feel cozy with your username, it will irritate you sooner or later. However, especially if you are still young or new with Internet, be prepared for that your naming taste may change later and you may dislike that name you picked first! (That's something I have seen in the comments of this tutorial.)
For example, my first online username was, if I remember correctly, my given name + my birth year (like, Lisa90,