TO UNDERSTAND THE RULES
The first things every player should understand are the rules of the game. At commercial fields the rules are usually presented at the start of the day. Knowing what constitutes an elimination, for example, can keep a player from leaving the field prematurely. Knowing the boundaries and time limits can help a player decide on strategy. Knowing that the people wearing orange vest are referees can eliminate wasting paint.
TO UNDERSTAND EQUIPMENT
While it is not necessary to be able to field strip a paintball gun, every player needs to know at least three things: 1) how to cock the gun, whether it is a pump or semi-automatic, 2) how to reload the gun and make sure the ammo hopper is secure and won't loose balls, and 3) where the "safety" is and how it works. Failure to understand any one of these concepts can not only leave you with a completely useless gun at the most inopportune moment, it can also be quite embarrassing when you finally figure out what went wrong.
Players need to communicate with teammates, whether by very vocal means, or simple hand signals. It is not necessary to develop a set of secret codes or signals when simplicity works fine. If you see two players from the opposing team on the hill ahead, get your teammate's attention, hold up two fingers, and point at the hill. Chances are, he/she will not mistake the signal for your opinion on how peaceful the hill appears.
Failure to operate in conjunction with your teammates basically leaves you as a group of very lonely individuals. This is especially a bad thing when your opponents are actually acting as a team and tear through your ranks one player at a time. Team players support each other with cover fire, communication, and by working for a common objective. Even people who have never met each other before game day can work together - really.
While teamwork is a great idea, is does not imply that you have to hold hands with your teammates. Getting too many players in one position offers the enemy a large, tasty target of opportunity, while concentrating your own firepower and defenses in a limited area. It doesn't matter if the tree or bunker is big enough for three of you - piling in behind it only offers an aggressive opposing player an opportunity to ruin your day times three.
TO EFFECTIVELY USE COVER
If the tree is smaller than the diameter of your thigh, you might want to reconsider the location. If the bunker has lots of holes a ball can get through, don't think an opponent won't spend his/her time and balls trying to make it happen. Got a solid bunker? Good. Now stop looking over it and presenting your opponents with the big target of your forehead and skull. Look around the side of the bunker.. peek out with one eye while keeping the rest of your body covered. Pull that elbow and that knee back in there - some people will shoot at dang near anything.
There is a whole game out there despite the fact that the player shooting at you has a seemingly endless supply of paint. While defending yourself from that player is important, also realize the player's buddies are using your fixation as an opportunity to move on you. You must stay aware of your surroundings at all times. Fixating on one target only serves to distract you, and create a smile on the face of the player sneaking up on you.
Paintball guns have a limited range. Most commercial fields have a target range where you can check a gun's performance before you play. Whether precipitated by panic or over-zealousness, one of the most common newbie mistakes is to fire at an opposing player long before there is a snowball's (or paintball's) chance in hell of hitting him/her. This is an especially bad thing when the opposing player hasn't even seen you yet. If an opposing player isn't aware of your position, wait until he/she is as close as possible before firing. The chance of hitting targets increases exponentially with every yard the distance decreases.
FAILURE TO BE AGGRESSIVE
Did you come to play or merely to survive? There is no point in playing a totally defensive position in paintball. Besides, the best defense is a good offense and all that stuff. In paintball, there is usually an objective, like capturing a flag. You can't do that from way back there. Having trouble reaching your opponent with paintballs? You can't do that from way back there either. Move up. Get on the trigger. Take the game to them. After all, it's only a game, and the worst thing that will happen is you'll have to come back and try again next time.
PREY TO INTIMIDATION TACTICS
Shiny paintball guns DO NOT perform any better than their dull-looking counterparts. Anyone with money can buy a flashy jersey. Get honest with any experienced player and he/she will tell you that a lot of the flash is exactly for the effect it has on you - intimidation. Don't be fooled. The most effective player on the field may be the guy standing next to you with a rental gun - but that doesn't matter either. What should concern you is what you do when the game is on. Now, what are you going to do? Learn the rules, understand how your equipment works, communicate and work with your teammates, look for good cover but don't bunch up, don't fixate on one person, don't fire too soon, but be aggressive and use firepower when you need it - and look at the player with the shiny gun and just say to yourself "you're mine!"