Make it Personal
Use something personal about yourself. You can use a nickname people have for you or base it on a hobby or activity you like. For privacy reasons, your user name should not give away any information that is too personal, such as your full name or your birthday. Also, if your goal is a unique user name, you shouldn't use your first name, as there are already thousands of Micheals and Johns out there.
Play Style
Base your user name on your style of play. If you are player who typically uses the sniper class, make your user name that includes something like "Marksman," or "Bullseye." If you use a name that hints at your play style, you risk other players getting wise to your strategies. If you are proficient, however, you can make yourself feared on the battlefield.
Show Your Interests
Using something that reflects your interests, such as a pop culture reference or quote as your user name can help you make friends online. Personal interests, such as the names of cartoon characters or your favorite sports team, will tell people your tastes up front. If you use something general and universally known, likely a lot of people will tell you their feelings on it. If you use something that is less known, you can find people of similar interests and play online together.
Make Them Laugh
Having a funny or clever name will help your gamer tag stand out among other players. Try to think of something short and clever that will make people smile when they see it. Remember that your target audience is people who play online multiplayer games, so try and think of a quip or joke that will appeal to that crowd.