For beginners, start with a 3/4 pot-sized bet. That should be your standard bet. If the board is scary (straight or flush draws likely), bet more. If the board is ragged (no obvious draws), bet less.
As you get more experience, you'll be able to vary your bet-sizes even more, but a 3/4 pot-sized bet often ensures your opponent, if behind, will either fold or make a bad call because he/she won't be getting the right odds to call. Still, it isn't such a significant amount that your opponents won't make mistakes and it doesn't turn your good hand into rags because only a monster hand would call you.