Here are some tips and strategies which have worked for many many many times.
1) First and foremost, always buy a Kevlar vest at a minimum, when you have plenty of cash ALWAYS but Kevlar vest and helmet, if ct, allways buy a defuse kit to start with, then weapons.
2) LEARN the keystrokes for buying the items you want, know the costs of the items, and build the save keys for your auto-buys.
3) Buy different guns, learn which you prefer, and learn how to use it, not all guns are the same, some are very effective when fired quickly, others are not, and always buy the one which fits your play style best.
4) If you are just starting out, and you don’t have aiming down, get a good SMG or shot gun along with your gun, and use cover to your advantage, use a wall or doorway and hop out, fire, retreat.
5) Along the same line as #2 use smoke, flash and regular grenades as efficiently as possible. Use cover smoke your area, flash if you see a group of people coming, followed by a grenade, most inexperienced players will stop dead in their tracks, and a well placed grenade will kill them or hurt them very badly, SO..make sure you don’t flash yourself, and follow your flash with a well placed grenade. REMEMBER the closer to someones feet the grenade lands the more damage it will deal (93 Dmg is my personal best)
6) About grenades, if you don’t know how to aim with one, DON’T USE THEM. It will only get your teammates mad, and if FF (friendly fire) is on, I have found out from personal experience, your own teammates would rather kill you then be flashed again…use bots, and make your PC a server and play vs bots first, before using them in-game.
7) Use many smaller teams to flank your opponents, when you use voice comms, it is best to use several smaller teams, and flank your opponents, have a small team 2 or 3 people guard a major choke point, with all members holding smoke, flash, and grenades, and time them 2-3 seconds apart, by the time the last flash has worn off, the team who went to flank your opponents should be taking fire from both sides.
8) ALWAYS use hotkeys, make several hotkeys, figure out what weapons you use the most, so that you do not have to scroll to change weapons, one second can mean the difference between life or death.
9) Along with using hotkeys, use hotkeys for different nades, know which are which, these can save your life, or at the very least help you take an enemy to the grave with you, helping your teammates in the process.
10) Use your flash nade key as SOON as you get flashed and throw it in the direction you were flashed from and then turn your vision into the wall, the seconds you are blind can be over-run by your own flash from someone coming towards you, and most people will retreat instead of taking a chance of dying from being flashed themselves.
11) AND THE LAST tip: NEVER, EVER, be the first person through a choke point, you will take all of the fire directly, and only end up counting up your deaths, I prefer being third, the first person almost always dies, the second person makes the person or persons on the other team retreat, and you get to help clean-up!