When you join the server, notice which side has the fewest players, then join that team. The way you do that is from the player setup menu. Hit Escape, scroll down to Player Setup, scroll down to Team. Hit Enter until you have selected either the blue or the red team.
The opposing team may capture your flag while you are away from your base. If this happens, you will not score a point until your flag is returned to your base. You will continue to carry the opposing team's flag either until you are killed or until you bring their flag to your base while your own flag is there.
Even if you capture the enemy flag, you may be killed before you can return to your own base. If you are killed while carrying the opposing team's flag, one of three things will happen:
In CTF, you have two equally important primary objectives:
Suicide can be used to your advantage. While it may cost you experience points (if you happen to care about that), it can get you back to your base instantly. You can intentionally kill yourself with a rocket or jump into lava, which will cause you to respawn back at your base. Obviously this won't help you if you're the one carrying the flag!
Good defensive work is essential, but all of the defense in the world will not score points for your team. The most effective approach is to apply constant, unrelenting attacks on the opposing team. The opposing team cannot execute an effective attack if they are too busy defending their base. The more aggressively your team attacks, the less resources they will have available to use against you.
Stay out of the flag carrier's way! When you see your teammate running with the opposing team's flag, get out of his way! There is nothing more frustrating than running into one of your teammates and getting killed because your teammate wouldn't get out of the way. You should step aside and be prepared to defend your team's flag runner as soon as he passes. You can be sure that if your teammate has the opposing team's flag, the rest of the opposing team will not be far behind.
Learn the map's layout and utilize its features. Some maps have doors, lifts and other tricks. Some of them can be used offensively as shortcuts, or defensively as obstacles against an enemy attack.
The moment you spawn, race toward the opposing team's base as fast as you can. Do not waste a lot of time gathering up weapons. Grab the nearest weapon and may be one heavy weapon. Grab any armor or extra health. You'll need that to help you survive the trip to the enemy base and back.
Try to avoid enemy contact on your way to the opposing team's base. You will almost certainly encounter a battle at the opposing team's base. Try to run straight toward the flag, grab it, and make a quick getaway.
If the base is heavily defended, don't stand and fight. Find the quickest exit and make lots of turns if possible. This will prevent any rockets fired from the base defenders from hitting you. It is pointless to stand and fight at the enemy base if you have their flag. Every enemy you kill will respawn right back at their own base anyway. You physically can't eliminate all of them.
If someone else on your team grabs the flag while you are near the enemy base, you should stay close and try to defend the flag carrier. Try to block the passage using yourself as a shield, while firing constantly in the direction of the oncoming enemy. You're not trying to kill any one enemy, but rather slow down any enemy that might be in pursuit of your team's flag carrier. You do not need to stop them. You need to slow them down and weaken them. If they make it to your base, hopefully there will be someone there already to deal with the weakened enemy. It's easier for the base defenders to stop an enemy advance if you have already weakened them. Remember, as your teammates die they will respawn near your own base. Let them finish off the enemy as they approach.
If the opposing team has captured your flag, the top priority is to get your flag back. If the enemy flag has not been taken, and you are close to the enemy base, it might be better to attempt to take the opposing team's flag. Even if they already have your flag, they cannot score unless they retrieve their flag from you. This buys time for your team too attack the opposing team's base and attempt to get your flag back.
If you get back to your base with the opposing team's flag but your own flag has been taken, you must hide! Gather up as much health and armor as possible. Avoid any firefight if possible. Let your team mates deal with enemy attacks on your base. Wait for your flag to be returned, then immediately run to your flag.
It's not always a good idea to recover your team's flag from a downed opponent. If you frag an opponent in a way that leaves your team's flag hidden, maybe leave it there, especially if there are lots of opposing players near your home base waiting to pick up your team's flag again. Of course, if your team has the opposing team's flag and is waiting for your team's flag to be returned, then return it ASAP.
If you respawn in your base and one of your teammates is hiding there with the opposing team's flag, don't stay at your base to defend him. Some people will disagree with this, but consider these facts:
You have more to lose if you attempt to defend your teammate. Eventually he will be killed. If your flag is not at your base when that happens, the opposing team will very likely score a point. You have a better chance of preventing them from scoring if you can get your flag back than you would if you stay to defend your base.
If you respawn in your base and you see an enemy attack while one of your teammates there with the opposing team's flag , then you should stay and fight off the enemy. Get right in their path and use your body to block them. Keep in mind that if you get killed you will respawn right near your base, so you'll have a chance to finish them off.
Before you join a server, check the balance of the teams and join the one that needs players. For example: a lot of games start off 3 on 3, but end 6 on 3.