I used to be terrible at CS, but my tip would be:
whenever you have to shoot, stop walking for a second to let the gun become accurate, pick off a shot or two, and then start moving again. Don't crouch unless you need a perfect shot and the enemy doesn't see you. Typically, shooting while moving is hopeless unless it's close-range TMP-age or shotgunning. This advice mainly applies to Assault Rifles. I'd use the silenced M4 - although each shot is less accurate and does five less damage, there's far less kick between shots, which means you can shoot about five or six bullets at a time instead of one to three with the AK.
Stick with your teammates for as long as possible. Stay near cover.
Oh, and always buy armor. It doesn't help a huge amount, but whatever.
Lastly, play on maps like fy_iceworld for a while until you get good.