Burst fire, always at Medium-long distance. Full spray and pray only at close range and when desperate.
To simplify Purchasing, hit F1 and take the Preset Loadout. If you Buy a Loadout that you prefer, hit F2 to Buy it again. Guns and Armour are top priority, later learn how best to use HE, Smoke, Flash grenades, before you Buy/Use them.
Hang back and follow your Team Mates for awhile. Let them rush into battle, then when they engage you have time to react/fight back. Always check the angles(heard this in a movie once!

), don't rush through doors without checking all nooks crannies. Watch the flow of the Game, often the enemy will keep repeating certain Attacks and take up certain positions each round. Try something different next round to counter it, this is the advantage of just dying and waiting, you get to Spectate and/or look around the Map for alternate positions/paths.
I have always been Average+ at CS, Positive Ratio, but most rounds I suck, lucky to get a kill every other round. Out of a dozen rounds I might get 1 or 2 rounds where I'll just totally own, not only surviving to the end, but getting 4 or 5 kills in the process, maybe even a Bomb/Defuse or something.
M1/AK are the best Rifles, P90 is an awesome weapon too, especially for tight corridor action where you can rush through straight into oncoming enemies mowing them down. I've seen people get 5-6 kills this way, it's sweet. Shotguns work well for that as well. Silenced Weapons lose power, but are awesome for stealth, well worth the trade off if you flank an enemy and want to avoid Health loss.