The three main things to remember when playing on defense is to get big hits in tackles, a strong ruck, and to “build the wall”.
Be aggressive when you tackle. When the other team sees a strong tackler, their hesitation can lead to your team getting the ball.
When a ruck goes down, getting low and having a strong body position will help in gaining possession of the ball.
In a defense ruck, there are two people, one on either side of the ruck, called the guards. These two players aren’t always the same people. They are the ones who are closest to where the ruck formed that weren’t part of the ruck themselves. In order to let their teammates know that the positions are filled, they raise their outside the ruck side hand and yell “guard! guard! guard!”. Their job is to stay right on the outsides of a ruck and watch for the ball to be out. Once the ball is out, they yell “ball’s out!” and run forward to tackle the ball carrier. Their unofficial position is similar to that of the flankers in a scrum.
If it looks like the other team is winning possession of the ball, the team will get into a flat line behind the last foot of the farthest back person in a ruck. Stay spread out across the field and avoid having big gaps in the line.