Good Advice From Penny Arcade
Penny Arcade is a great online comic, and they are also full of great advice. As this drawing, and shirt image, illustrates: when you are playing Team Fortress 2 you should focus your firepower on the guy healing the guy killing you... Here is the original comic that inspired this shirt.
On a related note, Team Fortress 2 is an awesome game. If you find yourself with some money to burn, you owe it to yourself to pickup Valve's Orange Box. It is chock full of awesomeness. Inside this one box you will find Half Life 2, Half Life 2: Episodes 1 & 2, Portal (The Cake is a Lie...), and Team Fortress 2. As a bonus, you get a taste of Peggle with Peggle: Extreme. Try to resist buying Peggle: Deluxe (the full meal) after this little appetizer. You can get Orange Box online via Steam, or buy it in stores if you need to display a box on your wall to show all who enter your abode that you are rocking the latest in gaming goodness.