After seeing a lot of new airsoft folks being "untactical" or not knowing what certain tactics are, I decided to right a guide on some basic stuff. Once you get your foundation down, your set for any light MilSim game. Let's get started.
Just like in football, there is basically two "statues" you and your team can be in. "Out contact" and "In contact". I know these aren't technical terms but you just need to know about them, not their specific name.

"Out contact" is when you are not directly facing the enemy, AKA not taking or engaging fire. While this usually never happens in a pick-up game, in a MilSim scenario, it is bound to happen. Usually you aren't in an engagement when you are patrolling, defending an objective, or various other task. The main thing to remember is to be stealthy.

"In contact" is when you are fighting, AKA in a firefight. This is usually the glory of airsoft and you will be in the situation more when your in an ambush, attacking an objective, or various other task. Main thing to remember is to be loud and make you and your group appear to be larger then it really is. Now let's get into a lil more detail.

"O.K. so I'm not firing...so that means I should be stealthy...how?" Glad you asked.
First, be quiet as possible and soak in your environment. Keep at least a 5 or 10 foot spread between you and your buddies (you should still be able to see them though). Look around as much as you can in a slow, methodical movement. You should be looking for OpFors (bad guys) and at your teammates. If you see a OpFor, here's a step by step on what you should do:
1. Make sure he's a bad guy. If he isn't, whisper to your leader and/or teammates "Friendly front/back/left/right".
2. If he's a bad guy, don't shoot! The other team may be baiting and you have more to loose if your wrong. Look around and see if any your teammates are looking at you. If they aren't, whistle or another nature like call to get their attention. Once you do, point your middle finger and index finger and point to your eyes then on your fingers show how many guys you see (this means "I see 1/2/3 OpFors") If you see one of your teammates do this, repeat the same motion. Make sure everyone knows.
3. Get in cover and wait.
You should always take these steps when you see an OpFor. It much better then yelling, "I see a bad guy!" or instantly shooting the unidentified personnel for obvious reasons.

While we're already talking about hand signals, here's some other you should know (only repeat these, your leader should be making these decisions not you).
Move out: Hand is brought behind, usually palm facing up, and then brought forward to the front of the body. If the leader's repeating this, it means he's pissed or you need to book it.
Stop: Fist is brought to head level and held up. I've seen people just hold up their hand like a high five, too. If you see someone do this, repeat it and then stay still and quiet. Usually this means a teammate hear something.
Get down: Hand is brought to chest level and brought down. Repeat this motion then get into cover as low as you can. If down quickly, usually means "Get :censored2: down".
These are the most typical signs I've seen. Again, the one thing to remember when your "out contact" is to keep quiet and stay alert.

Now you usual, you don't stay "out contact" for very long and sometimes never. If your making a transition from "out contact" to "in contact" (you got ambushed/attacked/ screwed), yell as loud as you can "Contact front/back/right/left!". Now that you got fire, you don't need to be quiet and should make your self look "bigger" (moving from cover to cover and actually firing at the enemy). When you initially come into "in contact", you want to make sure your in decent cover and you know where the enemy is coming from. Once you do, stay on auto and fire because you need to gain fire superiority. Ultimately, you should be doing "The Four Fs" which are:
1. Find where the enemy is. Make sure you know if their on one side or two or three.
2. Fix them which means get fire superiority or pin them down. This is the time where you should be going auto and firing where you see any bad guys. Make sure the bad guys either get killed or get stuck to a piece of cover.
3. Flank them. This is where you want to go to semi and fire one to two shots every second. Enough to keep them in cover and enough to conserve ammo. If your playing on a smaller field or with a smaller group of people, you and a buddy can coordinate a plan to go around the guy and get him where he's weak. If your playing on a bigger field or with a bigger group of people, more then likely you and your squad will need to work together and get these guys.
4. Finish him. Kill him and make sure he calls his hits. If there's medics and your in a good spot, stay around cause more people are most likely going to help him. I've gotten five people once cause I shot one guy.
Now if you can't get fire superiority or the force is much larger, a smart leader will know he needs to get out of there. Basically, you will be retreating one at a time and not all at once (called "leap frogging" or "peeling"). When you and your team falls back and it's your turn to run, say "Moving" or something along those lines. Keep going until you see the end of the line and go 10-15 feet behind the last guy. When your there, yell "Set!" and the process will be repeated until your a safe distance away.

Let's review. There are two positions you can be in, "out contact" and "in contact".
While "out contact" you should be stealthy, looking at your surroundings and being quiet. Look for OpFors and teammate signals. If you see a guy not in your squad, go through the steps. Repeat hand signals if you see your leader doing them and then follow what they say.
While 'in contact" you should be loud and try to achieve "The Four Fs". If you can't do that, fall back by "leap frogging".
Hope this way helpful and keep in mind this is only the beginning.