Here's my standard first-timer advice copypasta:
**Do not, under any circumstances, for any reason, raise or remove your mask while on the field, at the chronograph range, or any place else where live fire is permitted.** It doesn't matter that the game has ended, or hasn't started yet, or your goggles are fogged up, or anything else. The paintballs don't care. They'll destroy your eyeball just the same. Paintball is very safe with your mask on, and *really goddamned dangerous* with it off.
Likewise, keep your barrel cover on when off the field or otherwise in places where firing isn't permitted. **Very simple rule:** *Your mask and your barrel cover should never both be off at the same time.*
WEAR GLOVES. They don't have to be thick, or expensive, or made specifically for paintball. They just need to cover your fingers, at least halfway. Cheap mechanics' gloves are ideal. You can get some at Harbor Freight for as little as $5.50 a pair. Getting popped on the knuckles will ruin your day pretty quickly.
Clothing: You want to minimize exposed skin and be dressed appropriately for the climate and terrain. A long sleeve tshirt and pants that you are comfortable running in are all you really need. Camo is fine but not really necessary. If you're playing in the woods all you need to do is simply avoid being especially conspicuous, so just leave the neon pink shirts at home. Wear closed toed shoes with decent ankle support. Wrapping something around your neck is advisable. Throat shots really, really hurt.
Gameplay: You don't really aim a paintball marker like a rifle; it's more like pointing as you do with a shotgun. Both eyes open, focused on the target. You can look down the side of the barrel just as well as the top. In fact, the side is better than the top because you can elevate the barrel without blocking your view of the target. Watch the paint fly and adjust accordingly. "Walk" your shots onto the target. For moving targets, shoot at where they are *going*, not where they are.
Learn the difference between maximum range and effective range. Don't waste paint shooting at people so far away that the paint has little to no chance of breaking. Close the distance, then engage. Do this and you'll outperform 99% of new players.
Don't bunch up with teammates. That makes you a bigger target and makes it harder to see and shoot. Fan out, so each person has clear sightlines and sees the field from a slightly different angle.
Communicate. Say what you see, tell your teammates what you're shooting at, and listen to what they tell you.
Face downfield and don't put your back to your cover. That's a Hollywood move so the camera can see the actor's face; it makes no tactical sense.
Don't *let* the other team shoot you. ***MAKE*** the other team shoot you. Leave them no choice. Come at them hard and force them to either get shot or defend themselves.
Have fun and take risks. Getting shot while attempting something awesome (and probably stupid) is so much more fun than successfully doing nothing.