Tap the play button in the main screen. This will take to to a tier menu, select the tier and prize you'd like to play against. Remember the higher the tier, the bigger the reward but you play against better players. Once in the game it's 8 ball pool rules:

The object of pool is to pot all of your designated balls (either stripes or solids) and then pot the 8 ball, thus winning the game. As pool matches often consist of several games in a ‘best out of’ format, players attempt to win as many games as needed to win the match. Players must use their skills in both attack and safety play, as well as tactical nous to help them win the match.