I love realistic military games (as you might have guessed) and have converted a few people to buying ARMA 2 (see impressions here)

Now if you try to play ARMA 2 like COD or something similar you will die. Very quickly.Instead, you should think of yourself slightly differently. Here’s a few pointers:

1. You are not God. It takes a single bullet to kill you. Now don’t say “headshot kill in one in COD!” to me. In ARMA 2, one shot to a vital organ and you are on the ground, bleeding. A round in the head or lungs, and you can kiss the idea of completing a mission goodbye.
2. Guns are not your friends. In other games, bullets fly exactly where you want them to. In ARMA, bullets act like real bullets. Expect them to drop slightly when fired and at long range be affected by the wind and eventually the coralis effect (ask MacMillian about that)
3. Don’t be a lone wolf. Unless the mission forces you to be, you need to follow your squad leaders commands. Act together with your squad members, and you will be fine. Well fine-ish
4. Be smart not stupid. If your hid behind a wall and thinking of running towards a haystack, think about what you are doing. If there is a MG spraying the wall above you, don’t stand up.
5. Learn how to play before going online. Go messing around in the editor with all the different peices of kit, especially personal weapons. Learn how to shoot assault rifles through iron sights and reflex sights. Also learn how to snipe (its a bit harder than in COD) and use AT weapons to knock out vehicles at 300m. Once this is done, you can easiler jump in a game.
6. Use VOIP. Anyone who has played in clans in COD games will know about how useful this is. ARMA has VOIP built in. Its easier to shout “SNIPER” than to frantically use the crap ARMA interface
7. Lean!!! If your in cover, use the oft-ignored lean keys. Rather than sticking your entire body out, just put your gun out
8. Vehicles without infantry support die. Wait for infantry if in a tank or provide support to the angry houses
9. Read this full guide. Some points are not useful (like those about ACES  2), but most of it is brilliant

I intend to do some more on this guide at some point, but this is a good introduction for anyone coming onto ARMA 2 from other FPS games