I posted in your thread in the Online gaming forum, but from this post it sounds like you need more basic lessons. :-\
About defenses: In general, but not always, you should pick a defense that matches up well against the formation your opponent choses. 4-3 and 3-4 are good, balanced formations to use against balanced offensive formations. When your opponent chooses his play, you'll see something like 2WR-1TE-1RB, or 4WR-0TE-1RB. You should key off of this information, and atleast have as many cornerbacks on the field as they have wide receivers.
Here's a brief guide.
4-3: 4 down linemen, 3 linebackers, 2 corners, 2 safeties. This is a good, balanced defense.
3-4: 3 down linemen, 4 linebackers, 2 corners, 2 safeties. An alternative to 4-3, good for when you have an excellent linebacking core, or are getting burnt on outside runs.
Nickel: 4 down linemen, 2 linebackers, 3 corners, 2 safeties. Should be used when your opponent uses 3 wide receivers.
Dime: 4 down linemen, 1 linebacker, 4 corners, 2 safeties. Good against 4 wide, horrible against the run.
Quarter: 3 down linemen, 1 linebacker, 5 corners, 2 safeties. Good against 5 wide, or when your opponent has 4th and 35
