Shooting the weapons
When you start trying to increase your repertoire of viable guns, it's important to take into account their strengths and weaknesses and be sure to play to them.
Most guns are less accurate when moving. The sniper rifles are the worst offenders, so people develop techniques of getting themselves to stop moving exactly when they are about to fire. Some choose to tap shift the moment they let go of their move key, and others choose to tap the opposite direction. This causes you to stop instantly instead of requiring a slow down. Chances are if you missed that awp shot when you were right on him, it's because you were moving when you shot it.
Other rifles are slightly less inaccurate while moving. If you're shooting at close range, it's not too horrible to move around a bit while shooting. If you're trying to hit people at long range, though, be sure you're not moving while shooting as this will increase their accuracy.
Submachine guns and shotguns are meant to be shot while moving. You shouldn't be shooting them at long ranges (if you have such a weapon be sure you put yourself into positions where you will only encounter enemies at short ranges as much as possible) and they can be controlled very easily while moving. When using a submachine gun, a shotgun, or a pistol at close range, use a technique called circle strafing. (I couldn't find a good video for Counter-Strike, but this video from TF2
illustrates the concept.) When circle strafing, put yourself into close quarters with the enemy and strafe around them in a circular motion, and they won't be able to keep up. Circle strafing is a great close quarters technique and especially useful when using submachine guns with lots of bullets.
Pistols should be shot similar to rifles at long range and submachine guns at close range. Be sure to stop moving when shooting at long range, but at close range circle strafing is key.
All guns can be sprayed if you understand their recoil patterns
. However, you should always try to kill people in as few bullets as possible to reduce the frequency of reloading. Spraying should be limited to close quarters as spraying at long ranges is quite ineffective. Semi-automatic guns such as pistols and snipers should not be sprayed at all. The deagle in particular has a very extreme recoil and a very large recoil reset time and should be shot slowly and carefully. Submachine guns work best when sprayed but you should attempt to shoot controlled bursts at long ranges.
One big problem new players have is they will get nervous and start spraying, especially when an enemy is spraying at them. Remember to always keep a level head and to stay calm. If your gun doesn't work well while spraying, then don't. The fact that they're spraying at you just means you have even more time to calm down and pick your shot, because they probably aren't going to hit you.
Be sure that the weapon you choose matches the range in which you'll be fighting, and that you choose to encounter people in ranges that support your weapon. Don't buy an AWP if you're planning to go through apartments on de_inferno. AWPs are great long range weapons and terrible short range weapons, and apartments is entirely close quarters. Likewise, if your team had to do a save round and all has PP-Bizons, don't call a strat through outside de_nuke, as that is a very long range area.
Certain weapons compliment each other very well. Others do not. A team full of AWPs can't cover each other very well. Be sure you are buying weapons that work with the strat being called, as well - don't just buy a weapon because you like it.