Here are some general tips for Realistic Difficulty:
Upgrade and use your pistol:
It's going to be your best friend. The first shot fired is always incredibly accurate, and generally, the bullet will hit exactly where you're aiming. Making headshots quite easy. Your pistol also has infinite ammo, so don't worry about running out!
Hot Glue pointed out that the Five-Seven is the best pistol, as it allows you to mark four targets(When upgraded.)
Seek a vertical advantage whenever possible:
I can't stress how much this helps. Enemies generally won't shine their flashlights up at pipes, or onto roofs, so hide in these locations religiously.
Use your gadgets:
This may sound trivial, but I often forget to use these myself. Your gadgets will be fully replenished every time you interact with an Ammo Stash. Needless to say, all of them are incredibly helpful.
Take a shotgun(Or an AR):
I personally find that most of the other weapons are somewhat bad on Realistic difficulty. As I mentioned before, your pistol is incredibly accurate(And it has infinite ammo) so if an enemy is far away, it's easy enough to simply pick him off with your pistol. If several enemies are closing in on you, however, killing all of them with your pistol is going to be difficult. Which is where the shotgun comes into play. A single shot from mid to close range with the shotgun will kill any enemy. If you have the silenced shotgun, kudos.
Note: Assault rifles are also quite nice, so the choice is yours. I'd recommend avoiding any other type of weapon, though. The SC3000 assault rifle is a personal favorite. It's silenced, accurate at long range, and quite powerful.
Sonar Goggles, use them:
I shouldn't have to explain this one. They're crucial to your survival, and your enemy's ultimate demise. Mark your enemies when you spot them with sonar(You can spot them through walls once you've detected them). This makes it much easier to keep track of their movements without having to constantly switch on sonar.
Look for a light switch before shooting out the light. It's quite a bit more subtle.
Note: When you turn out a light, it may attract a nearby hostile, but he won't bring any backup with him! Use this to your advantage.
Flashbangs, and how great they are:
A fully upgraded flashbang grenade will stun enemies for 5 seconds. If you hit two enemies, you'll almost always have enough time to hand to hand kill both of them. Or, grab one as a human shield and throw him into the other guy. Then you'll definitely have enough time to kill 'im! Go on, try it!
Don't shoot everything!
Especially not all the lights! Sure, if no one's around, by all means, shoot the lights, give yourself a place to retreat to. If you're still clearing a room, however, shooting one or two lights is alright, but there's no need to get trigger happy. I've seen a lot of people get killed because they were a little too quick to shoot out that third light.
The Portable EMP:
Is by far your best gadget. It has quite a few uses. It's an instant activation with a huge range that knocks out all lights and stuns enemies. My personal favorite use is entering a room undetected. Find a well lit room with 5+ enemies inside, two of which are patrolling around the door? EMP them! The (flash)lights will go out(Making you invisible) and the enemies will be stunned. The lights will stay out long enough for you to find a suitable bit of cover.
Naturally, it also shines when you're trying to escape.
Hope this helps!