The rules of a heads up poker game are similar to that of any other poker game.
The main difference between a regular poker game and a heads up poker game (besides the number of players) are the way the blinds alternate. In a heads up texas holdem game, one player has the dealer button and is the small blind and the other is the big blind. The player who is on the button is dealt the first card. This alternates every hand.
Each player is dealt two cards face down. The player who is the big blind starts the betting. After the first round of betting, the flop (three community cards) is dealt. Now the player who is not on the button is first to act. After this round of betting the turn (fourth community card) is dealt. Another round of betting ensues in the same fashion as before. Next the river (fifth community card) is dealt. Another round of betting starting with the player who is the big blind before the showdown. After this round o f betting is complete the players show their cards and determine the winner.
Remember that a player can fold to a bet anytime they want, thus ending the hand early.