Teach me please

Can someone explain me about using "at" "for" "to".
For example: I come to work. I come at work.
I say it for you. I say it to you.

I come to work. Now I am at work.

I say it to you. (you could say "I say it at you," but this sounds like you're being rather aggressive.)

Generally, we use "to" when talking about motion (going, coming, etc.) and "at" when talking about location (often with the verb "to be:" He is at home.  "At" can be used instead of "to" if you are attacking someone, for example, you shoot at someone, or throw the ball at someone if you want to hit them. But if you want them to catch the ball, you throw it to them.

A more difficult question would be when to use "in" instead of "at" when talking about location....