[Bona]

Value bet when you are ahead and expect to win the pot more often than you will lose it.

As presented your question is far too broad and spans multiple concepts. A simple example of a value bet is raising AA preflop. You do that because you believe your hand is best and is likely to stay best often enough for it to be a +EV (expected value) situation. From there it devolves into more complexities but basically you bet for value when your hand can reasonably be expected to take more from the pot than it puts in.

There are a number of concepts you should look into that will help define your questions. There are starting hands charts that estimate the value of your pocket cards. There are books and articles that teach what situations to play and why- that will help you know when you are in a +EV situation. Click on "Books" in the sponsored forums column on the left.