Try to walk ALOT. You'll want to be walking most of the time in combat unless you're running for cover or standing still, which is probably a bad idea. The slower you move, the more accurate your shots are, but the easier you are to hit. Find a good compromise and set up your playstyle from there.
Also, make sure you're not buying shitty guns. The MP5, M4A1, AK47 and P90 are probably your best shots in terms of price and efficiency. Try not to spam with the autoshotgun, it's probably better practice to use the M3 anyway. Make sure you know what you're doing if you're rushing in with an autosniper, light SMG or M249. Once you get better with those first four (or five if you use the M3 alot), then it's probably better to move into other weapons. Don't snipe too much unless you do it alot in other games, no need to hinder your team by standing still and getting shot.
Oh yeah, and while cover might not seem very important in a game like Counter-Strike, USE IT. Use it lots. You can absolutely decimate the enemy team if you utilise cover properly. This goes for pretty much any FPS, come to think of it, no matter how open and fast-paced they get.
Learn to make distractions, too, like smoke-grenading areas nowhere near you or flashbanging people at the right time and place. That's certainly useful if you need to get somewhere fast, or distract a sniper, or sneak up or whatever.
Oh yeah, and try not to get into the habit of constanly changing weapons. If it helps, bind mousewheel down to jump and mousewheel up to fire, and just use 1, 2, 3 and 4 to change. This'll get you less inclined to change and quite often save you from a surprise encounter that you otherwise would've lost from just not having your weapon out.
Well, there's my set of tips. Go nuts :>