You can find the full FIT rules here, but you probably don't want to read them, so here is a simple version, which assume you know roughly how the game works. (Alternatively, if you know what rugby league is like, then touch is just like 6-a-side league, but without kicking, tackling, scrums or lineouts.)

1. There are two teams of six players, and they play on a pitch measuring 50m wide by 70m long.
2. One team starts off by taking a tap from the half-way line.
3. They run forwards, passing the ball to each other, until they place the ball over the touchdown line at the far end of the pitch, or they lose possession to the opposition.
4. Every time a player is touched by a defender, the touched player places the ball on the ground and rolls it back to one of his team-mates, who picks it up and passes it to another team-mate.
5. Once a team in possession of the ball has been touched six times, possession is given to the opposition, who continue play with a rollball at the position of the sixth touch.
6. You score by placing the ball over the touchdown line, scoring one point. (This is not really a “try” because there’s no conversion, but people generally call it a try anyway!)
7. Game Over!