I know people can sometimes get frustrated as enemies will fire at them from off screen. Sit back and chill; if you realize, enemies aren't that far off screen, and you probably made yourself an open target.
To maximize the range you can see offscreen, instead of heading straight up, down, left, or right on the screen, rotate the camera so it's on an angle. IE: you walk towards one of the four corners of the screen to progress.
You don't have to use it all the time, but when you use it you can see just a bit farther, and can locate some of those harder to see enemies.
Grenade can cause enemies to turn away from you and towards the explosions. Use this to sneak up behind an enemy and attack!
It's still possible to 'dumbfire' or 'blindfire' your weapons to attack enemies too far off, or angled strangely so you don't have a lock on. Most accurate would be pistols or preferred, rifles.
The stealth skill can help you make less noise and therefore get the jump on some enemies, but also note that you must keep low and walk as slow as you can even when just scouting the area. Keep your gun aiming and rotate left to right in a loose arc; you may draw a bead on an enemy before he sees you - and if you creep along towards him, you may be able to take him out before alerting any nearby guards.
Hand to hand should be the last thing you raise. Getting that close to the enemy is always the least safest and riskiest method of attack.
I suggest raising the skill of the rifle category as fast as possible. Rifles are the most plentiful in the game, and they are also more accurate and stronger than SMG's per shot. They also have a longer auto-lock on range, and will help you snipe enemy troops in the field.
Then upgrade your medic skill. Eventually you'll be able to heal over time, and also heal a lot with just a small medpack. These basic skills covered, you can withstand the brunt of enemy attacks while further specializing your character whichever way you please.
(I suggest enhancing the SMG skill next, then grenade, then stealth, then pistol, then heavy smg, then whatever's left)
There are a great and many things in this game that can be used as cover. When you aim, if you get close to an object your reticle will start moving closer to you- this signifies that the object is blocking your field of view, as well as the enemies' if they are on the other side. Some objects require you to crouch a bit to get coverage.
Also note, that while you are covered here, you aren't fully covered. An enemy with high skill in a weapon class can sneak in a shot here and there. Not too bad for smg users, but can be a devestating hit for rifle users. Also, enemies may throw grenades overhead at you, so be prepared to act fast/move from cover to cover or blitz an enemy with a grenade of your own or scatter fire.