Well the most important thing i was ever told, by one of the top CS players was: 'Play Smart'. Choose the right weapons for the job, try to think what other players might do and what they just did. This way you sort of get to know where players will be. It's a little hard to explain, but i'll try:
example:
A CT sniper just took out 4 people down the pass on dust, if he's not too cleaver he's gonna be in the same place next time. So if you can't afford a rifle don't go down there.
You see what i mean.
It's sounds a little far feched, but you really have to think about what the other players are likely to do, by what they did last round. You see what i mean.
I apologise if you just wanted to know simple things like fire though boxes etc. Don't laugh at me but 'Play Smart' was the most important thing I was ever told.