It would be nice to have a formula for how much to bet. There are so many variables involved, it's impossible to cover them all here. For instance, is it a ring game with set blinds? Or is it a tournament where the blinds increase incremently throughout the event? What position are you in at the table? Are your opponents playing very tight or really loose? How large is your stack? What is the strength of your hand relative to all of the above?
I strongly recommend some reading on poker fundamentals, which will address betting in a lot of different situations. My favorites are Doyle Brunson's "Super System" & "Super System II", Tom McEvoy with T.J. Cloutier on "Championship No-Limit & Pot-Limit Hold 'Em", and "How to Play Poker Like the Pros" by Phil Helmuth. But there's a lot more great poker books out there.
Of course, the greatest teacher of all is experience through repetition. Just get in there and play!!