Each attacker starts 5 feet from a ball on the floor, on a whistle they do whatever they can to touch the ball, each defender does whatever they can not to let them. Switch offence and defence each time.
Get low, the players that win the battle are the ones who stay low and move their feet. On offence, you expect to be boxed out, try to get around him, make a move, e.g., jab and go opposite, swim move, spin off (don't fight the defence).
Winning the rebounding battle is an incredibly significant stat, in the same way as shooting percentage and turnovers. Key points on defence are 1) you have to care, want the ball, 2) be disciplined, you can't just run to the ball.
When a shot goes up, yell "shot" and box out - find your man, hit him (make contact with a forearm, try to move him back), go get the ball. It's an aggressive move, not a violent one, most guys will back away next time, send a message early. Half of the time we don't box out, and if we do box out, most of the time we turn our head and assume the guy is going to run into us.
See Rebounding - Circle block-out, 1 on 1 block-out, Jr. NBA - Rebounding.