Never stop moving. Ever.
Set up your keys and KNOW where your keys are for all the functions you'll need.
Practice, practice, practice! Remember, many online gamers have been playing FPS games for years. It'll take lots of practice for you to get to that level.
Learn to have fun even when you're getting the snot kicked out of you. It's just a game. If you enjoy it, and keep playing it, you'll get better. Getting your but kicked can be fun if you're just playing the game for the fun of playing.
Practice in single player mode. Set the bots to be much better than you are. Die a LOT. If the bots aren't reaming you, you haven't set up a challenge for yourself.
Learn the weapons. Some weapons are better for close-range shots, others are better for long range shots. You need to know which is which and be able to choose the weapon based on the target.
Learn the maps. You need to know where the weapons, health, powerups, etc. are.
The bottom line is that you won't become a world class player overnight. Some of the players you're competing against have nothing better to do with their time than spend hours upon hours upon hours of FPS gaming. These guys tend to get good at it. Unless you want to sacrifice having a real life, deal with the fact that these pathetic losers are going to be better at the game than you, but couldn't get a date with a girl if their lives depended on it.
So it boils down to two things: practice, and have fun.
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