Defense eh? Hrmm ok well then.. most people have already filled you in on most of the tips. Don't be afraid to take 2 steps back to take 1 forward. What I mean is don't be afraid to make a back pass to your keeper. That's what he's there for. If you unsure about making a square or forward pass..push it to your keeper and let him observe the field and decide where it should go.
also, defenders don't always have to stay back..they can make a run up the side or up the middle to open up space or to carry the ball in further. In other words if you get the ball and you have 30 yards of open pitch, don't boot the ball away(unless there's a breaking player) carry it up the field and make the opponent come to you. that will open up other areas of the field. Hopefully a midfielder or your other backs will back you up
and as already mentioned. Be patient with the ball. When a person is dribbling people lunge to get the ball from you. Use this to your advantage. When they lunge go around them or pass. That's more of a striker situation but you'll have to use it on defense.
ok next, learn how to shield the ball with your body. If the ball is going out or you have an onrushing opponent, put your back to him and keep him there. This is generally used for sideline and goalline situations but can be helpful in midfield if you're making a back pass.
this next statement may cause problems for some. If you want to know how to play defense.. simply watch Manchester United games
you'd be surprised how much you can learn watching soccer games from around the world. It helped me alot when I was younger. Even to the point that I made junior nationals.. too bad I didn't pursue it. Anyway, check out some games watch field positions, run times, and footwork. You should be able to catch on pretty quickly that way.