Which formation to use depends on how imminent contact with the enemy is. In paintball contact is always likely so you probably can limit it somewhat. In general, if there's little chance of a flanking attack and you have room to do it, it's best to walk in a nice relatively straight skirmish line in order to maximize your forward facing firepower. If, you think that there's some chance of a flanking attack, but it's most likely that firing will come from the front, then you would want to use one of several different formations depending on the level of threat from the flanks. These all rob some degree of forward facing firepower, so it's important to try to make a good estimate of the chances of flank attack. If you're almost certain that an attack will come from your flank then you should use a "file" formation, which is just a skirmish line turned sideways, or if it's most likely that the attack will come from the side, but you want a little more firepower to the front "just in case", you could use a "double file" as opposed to "single file". At any rate, all of these formations are very fluid and should change immediately upon contact with the enemy to whatever shape most effectively places the most firepower to the front you're firing at. Usually this will be a skirmish line unless fire is coming from more than one angle (in which case you need to move ASAP either forward, backward or sideways to a better, easier to defend position). Here are some formations that direct differing amounts of firepower to the flanks during movement:
***ALWAYS*** No matter what formation you're using, remember
the first rule a newbie needs to learn. DON'T BUNCH UP!!! Keep several
yards between each player on the team so as to not present the enemy with
a "single" target
where they can kill several birds with one stone (or several players with very few paintballs as the case may be).
Obviously, the point is that you want to set up your unit so that you are least likely to receive enemy fire along the long axis of your line - where you have the most limited ability to mass your firepower (due to the fact that you might shoot your own teammates and you can't get any decent "angles" on your targets). The flip side of the same coin is that if you allow the other team to set up so that they're firing on you from down the end of your line you're going to lose people quickly since even if their paint misses the first guy in your line, it has a good chance of hitting someone else behind him.
Another thing to remember is that when you're advancing or retreating under fire you should always use proper "fire and movement" technique. Never have the whole team move at the same time while under fire. Have some number of the team (half more or less, but adjust the number as needed depending on the intensity of the fire you're receiving) firing large volumes of paint at the enemy while the other part of the team is moving. As soon as the first section of the team gets to the next good firing position they stop moving and begin firing, while the second section of the team moves up past the first secion to the next decent firing position. This leap-frogging continues until you gain your objective.
Finally, whenever it's possible, when your unit is attacking a position, it's best to use part of your unit to put as much fire on the enemy as possible from the front in order to keep their heads down and hopefully to create "target fixation" or "tunnel vision" on the part of the enemy so that they forget to look to their sides and rear. While part of your unit is doing this, the other part of your unit should circle around to one side or the other of the enemy position (ideally without being seen, but even if they are) and attack from the side or rear of it. This places the enemy defenders in a very highly undesirable position since no matter how hard they try to get behind cover they will be receiving fire from more than one direction. Their only choice at this point is to attempt to retreat (which would be tough, but the best option available to them), or stay in the "kill zone" and be eliminated. - Well, there is one other option: Surrender.