To capture a flag, you have to be the first person to arrive at a flag and wait until it becomes your flag. You can tell if the flag is almost yours by the ticker at the bottom of the screen. If it stops it means an enemy is nearby and needs to be eliminated before you capture the flag. If the ticker at the bottom is completely red, the flag is enemy, if the ticker is completely blue, it is your own flag. Eliminate all enemies and take the flag. You can not assist - you must capture the flag for it to count. Being in a tank can help protect you from enemy fire.