A pair of decent headphones are your friends. You can hear people approaching you and from which direction much easier than with surround sound speakers.
Try to stick with a team mate, much safer as you can watch each other backs and also provide cover while the other is disarming/planting bomb or gathering hostages.
The silencer for the M4A1 will provide you with more accuracy but will reduce the effectiveness of the weapon [ie does less damage per shot]. I tend to use the silencer a lot and only take it off if someone is quite a distance away.
The AK47 is amazingly accurate if you fire in bursts. The same goes for the other weapons but the AK can be scarily accurate.
Get friendly with the quick change. In a firefight you can't always reload and expect to survive, be ready to pull out your pistol and carry on blasting.
Try not to chuck grenades at your mates, same with flash bangs - it will make you very unpopular
Don't hesitate to pick up a fallen enemy's weapon, especially if it is better than your own.
Try to always buy a helmet with your armour - reduces the odds of getting a brain enema.
Ignore people who moan about you using an AWP.
Throw yourself off a bridge if you decide to use an Auto snipe.
The scout is the most evil sniper weapon in the game. Hard to use well but people usually don't realise they're getting shot!
When entering a new area check all the places someone could hide [as you play each map you will learn all the usual camping spots].
Before entering an enclosed space chuck in a flashbang or two, and maybe a grenade, then rush in and take out anyone who happens to be stumbling around blind.
Use your map for team mates before dobbing a nade.
If all but one hostage has made it to the rescue zone, don't hesitate to shoot him dead. The hostages are dumb and at some point at least one WILL get stuck on a box or be left behind. Aint worth risking the round to go and save one! Be careful with this though, some servers are bit arsey about people wasting hozzies, but as long as its only a one-off you should be okay
All of the above you will learn in due course and try not to get annoyed/upset if you spend a lot of your time getting killed. Just give it a little time and you'll get better. All of the above will come to you. Most of all have fun!
I haven't played in a while so I might start again later
SiriusB