Depending on your head coach, you should have a set of base defensive plays that are either 3-4 or 4-3. Right now your team will go into goal line/nickel/dime/etc. packages to match the offensive personnel, I'd like to give you more control over that and that's high on the list for updates.
You have to open the play card to see what formation it's using right now, but if you click on the play diagram in your game planning screen you can see what plays are from the base 3-4/4-3 formations.