Opening Hands and Position
In order to play a hand of poker, most of the time, you need something that is going to be competitive in relation to the amount of betting that has taken place. This is best accomplished when you are the last person to act—in this situation you have had time to see and digest exactly how everyone is going to play their hand. But what happens when you are one of the first people to act in a hand? You have a few options here, but your best one is to assume the worst.
By assuming the worst, I simply mean that you need to suspect that the people acting after you have good hands and will be betting them according to their strength. In this light, you should only play the strongest hands if you are first to act. This will ensure that you will not have to fold mediocre hands that you were hoping to play before you were raised out of the hand by someone with a superior hand.
I say that you need to do this most of the time because there is one exception. If you are playing against extremely talented players, you will (very rarely) want to bluff with a marginal hand. Good players all know this rule of position and will assume that you have a great hand if you raise in early position. This only works against good players though, as most people you will play against will not pick up on the fact that position dictates what you should play.