1) Don't stand in front of doorways or windows. Most likely, someone is looking down there.
2) Don't block your teammates. This is an easy way friendly fire occurs. Doesn't hurt to shoot in the same direction as your teammates too

3) When firing out of a window or doorway, give yourself an angle so that the least amount of area your body shows is minimal to any enemies looking through windows or doors. Also, helps to ensure that your gun isn't sticking out for the enemy to see.
4) When entering a room/door/window, it doesn't hurt to 'pie' that access point, meaning you check as much of an angle instead of waltzing right in and finding someone hiding behind a doorway to knife you.
5) If you're using a auto... to help accustomise yourself with the recoil difference between these autos in dod and CSS, aim for the enemy's crotch. Sounds weird but, the recoil will at least lead you up his torso when firing.
6) Communicate with your teammates and use your minimap. These ones are easy that you know from any game involving teamwork.
7) In realism, take it slow... as Smilo said, no point being gung-ho in realism (unless it's Attack vs. Attack). Patience and awareness get you far.
8) Learn the maps. Play on some of the eGO pub maps we have here. You'll learn a lot from the guys there as most of us spend our time on these servers when not in realism, OR you can track some of us down if we're going into public realisms somewhere to gain more experience. Knowing maps and surroundings will give you an advantage of anticipating enemy movements or positions.
9) Try keep yourself hidden from the enemy sniper. The snipers in realism are usually very good. Put yourself in a position that gives you the best possible advantage.
10) Don't forget that you can actually 'prime' nades in DoDs. It usually takes 5 seconds before it explodes so, priming it can ensure when you want it to explode and where if you throw it correctly.
11) Listen to your superiors and the others. They've got loads of experience and will guide you on what to do.
12) The Cake is a Lie.
13) HAVE FUN and Learn

Realism is a blast!