The round of betting before the flop is the single most important round of better in Texas hold'em. Because there are no community cards on the table, the only information you have is what you are holding in your hand. Since there are not a lot of cards, most people's hands are not particularly strong. As such, playing the cards in your hand is extremely important. Some people suggest that you can often limp in with a weak hand and hope to get lucky. Often, these same people suggest that you should try and trap your opponent when you have a really strong hand by just calling the blinds or making a small raise.
There are a number of dangers involved in this sort of play. The first danger comes when you are just calling the big blind with a bad hand. The chances are against you that you are going to make a winning hand and, depending on your position at the table, you may get raised and not even get to see the flop. That is why I suggest, especially for newer players, not to bluff pre-flop unless you are trying to steal the blinds.
On the other side, trying to trap people is a good way to lose many chips. By betting less than what your hand is worth in terms of strength, you will let other players in with hands that are not as good but could beat yours if they hit cards on the flop.
By folding your bad hands and protecting your good hands, you will be able to be making money when you sit down at the poker table.