In Hold-Em the first person to the left of the dealer posts the small blind, and the person to the left of him posts the big blind. In the first round of betting (before the flop), the person to the left of the big blind, or three left of the dealer (button) acts first. In every other round, the first person to the left of the dealer (small blind) acts first. It stays this way the entire time regardless of how many players. Its a little different when it is down to heads up. The button posts the small blind and is also the first to act.
