What do you guys think about when you raise after the flop to know the amount?
And on the turn?
River?

Like I raise half the pot after the flop, but I dont know what to bet when he raises first and I have a very good hand. 3 times his bet or something??

You need so much information to answer this question. As a general rule, if you're the preflop raiser you want to bet somewhere around 75% of the pot on the flop whether you hit it or not.

There are SO many factors in determining how much to bet. How many players are still in the hand, what cards are on the board, what hands you think different players may be holding, how tight/loose the players are, etc. Obviously one almost never wants to bet something irrelevent like 10% of the pot or something crazy like over 300%, but one really can't sum up how much you should bet in a general rule. All poker is is folding and betting, so you're basically asking one to answer "how do I play poker?" in a single post.

I do however, have a friend named Brett who is a total donk. I can try to find out his alias so you can watch him play for a while and you'll know to do the opposite. You used to be able to do the same for The_Golden_One at limit, but I think he gave that up and is pretty good at NL. Too bad.