Up-to-date rules can always be found on the England Squash website, but here are the basics:
The (singles) game is played between two players on a squash court each using a standard racket, and a ball approved by England Squash.
Matches are the best of 3 or 5 games, at the option of the competition organiser. Each game is point a rally scoring to 11 (PAR 11). If the score in a game is tied at 10-10, a player must win by 2 clear points.
Points can be scored by either player. When a player fails to serve or to return the ball, in accordance with the rules, the opponent wins the point. When the Receiver wins a point, they become the Server and add one to their score.