Beginners always make a few mistakes that everyone makes. I make them all the time. Here are a few helpful hints:
The very first rule is: never EVER take off your mask!!! I can't stress this enough. It is very detrimental to your vision's health. A paintball at 280 feet per second could pop your eye out of your head. Don't ever take your facial protection off, except in designated areas.
Make sure you understand your gun and its limitations, whether a rental or your own. Ask if you don't know.
Paintballs do not hurt when they hit. You won't even feel them till the day after. I have found that paintballs that hit you and don't break hurt a little more because they hit and bounce off. The time they are in contact with you is shorter and thus the force is greater (impulse equation from physics justifies this). When it does break it is in contact with you longer and is cushioned by your skin, fat, etc.
Wear darker clothes, don't wear white. White is just too bright, you will get killed.
Wear some type of boots or comfortable hiking/running shoes. I personally prefer football or soccer cleates. I think cleates work the best for me. They are comfortable, light, quick, and provide excellent traction in grass and mud. A lot of paintball players are using cleates these days.
Always be honest. If you get hit and see the splat on you, please raise your hand and exit the game. There is always another game to redeem yourself. I hate people who try to wipe their paint off and think they can get away with it. You only do that if you really suck. By the end of the day it will be obvious if you suck.
Don't ever call an advanced paintball player a cheater. They hate that and take their games really seriously. No matter what happens don't do this, you are asking for trouble. That is how seriously some people take their game. They are mostly very honest down to earth people. One time I did this and the guy wanted to fight me.
Don't be afraid to ask or learn. With the speed of technology these days it is hard for anyone to know everything about paintball.
Always appoint a team captain and follow his orders no matter what. This is the key. The team captain does not have to be the most experienced or the wildest or the most intelligent, but there has to be one captain. Follow orders even if you think you have a better plan. Don't go for self glory. I remember one time a young 9 year old played with us. The team captain told him to hide behind some tires and stay there forever until it got really bad or we won. It got really bad, our whole team was eliminated except for him. The other team had no clue about him and 3 of them came marching in to take the flag when he slowly jumped out and killed all of them. Obey your captain even if it does not conform to your method of play.
Keep your eyes open. I have seen so many players gain ground and then quickly get eliminated because they were not looking sideways. They go straight forward and then are attacked from other players from the sides and even behind them. This mistake is made so many times. It is a mistake even the most advanced players make. It's not easy to look around. Our brains always focus on one thing. Train yourself to look around as you run, hide, attack, and take cover.
Be alert and never focus too much on one thing. Don't keep shooting at the same guy behind the bunker over and over again. Move on and surprise him from the side or another angle.
Don't stay in the same spot too long. If you are in a fort don't keep shooting from the same side forever, its not productive. Go to the other end, go to the second level. Move away from the bunker walk out unprotected and hold people down. Don't stay put!
Never give up. Whether you run out of air, paint or energy... don't ever stop or quit, you are still useful. No one has to know that you are all out.
Never call yourself dead if you are unsure. If you question whether or not a shot hit you and splatted call a paintcheck and have the referee check if you are dead yet. Don't just call yourself out because you felt a hit. Wait for the referee to confirm.Always call paint checks on yourself if you are unsure.
Author Resource:
The Paintball Guru has an excellent Paintball Course with a lot of valuable paintball strategies. You should really check it out if you're serious about getting better at paintball.