If you're just learning to play, here's a good basic game to start with...
Step 1.

Establish a fort, or base camp, that is to be defended. A building or paintball bunker works best. The area must be something that gives the defending team an advantage, perhaps the top of a hillside or some place behind a lot of obstacles.

Step 2.

Place a flag in a central location within the fort.

Step 3.

Divide all your players into two teams. The teams should be even. If one side must be stronger, it should be the attacking team. Make sure everyone is wearing their paintball masks.

Step 4.

Decide which team will attack and which will defend.

Step 5.

Determine the length of each game and make sure each player knows. Also decide how the games will be started and finished.

Step 6.

Send the attackers to a predetermined place while defenders set up their defense.

Step 7.

Begin the game. The attacking team must penetrate the fort, capture the flag and return it to their starting point. The defending team must stop them or hold them off until time runs out.

Step 8.

Switch sides. Attackers defend, defenders attack.