I think defense is always harder to be good at than offense in Madden. Still, i consider myself a defensive player and that is where I find most of my success. The biggest tip i can give you is LEARN TO USER CONTROL a defensive player. I like to control a cornerback/Safety but since you are just starting out, i would recommend that you try user controlling a linebacker. It will take some practice and you may give up some big plays at first, but it will be worth it. You MUST learn to user control if you want to be truly competitive online. You will learn the routes and the plays of the offense, and will be able to predict where your opponent wants to go with the ball. Then you can jump routes and catch user interceptions!! Also, when you are playing defense, pay close attention to your opponents formation. You will be able to predict plays and make adjustments to stop it. It's just like making adjustments on offense to beat the defense, but on the opposite side.
Another tip that has helped me - Pick the formation you want to run for that play, and come out in a cover 4 play. That way you can audible to any of the base coverages in that formation (Cover 4, Cover 3, Cover 2, And 2 man under) Then after you have picked your coverage style, make adjustments from there.
Use all the defensive formations, and decide what works best for you. My defense is based on 3-4 (Specifically 3-4 Solid) but i like to pepper in a bit of everything to throw my opponent off.
Strengths and weaknesses
2 man under: good for run stopping since there is a defender that is manned up on the rb. The defenders jam the receivers up at the line, and this delays the pass. It will give you more time to get pressure on the qb, and more time to get where you need to be with your user defender. Be careful though, 2 man under can be easily beaten by out routes and corner routes. good for run stopping since there is a defender that is manned up on the rb.
Cover 2: Great for covering medium to short passes, and can throw your opponent off because 2 man under and cover 2 look the same before the play starts. In cover 2 you are taking the chance of giving up deep passes.
Cover 3: Again, great for defending short to medium passes and runs, but risk giving up deep passes. The difference is you only have 1 safety in the backfield instead of 2 like the you have with cover 2 and 2 man under. This means that you will have have an extra person in coverage for the medium passes or run stopping, but have only 1 person in the backfield to stop very long passes.
Cover 4: IMO the best coverage for stopping medium to long passes, but it gives your opponent the option to throw short dump offs and screens. Can be weak against the run as well.
Lastly, watch out for screens, seriously. In Madden, a screen can go all the way at any given time. You have to predict which side you think the screen will go to and then you can put one of the guys on the d line in a qb contain. He will be right there to stop the screen.