How to Mark an opponent

Marking is indefinitely the most important aspect in a defensive formation - while players would originally mark a certain individual (Known as Man-marking, most players do a form of zonal marking - going after people who enter their pre-determined "zones")
                                                     The Keys to being a Great Defender:
Keeping your eyes on your man and the ball
Keeping the player in front of you
Not letting the player turn when they receive the ball with the back towards goal
Closing down fast when the ball is in the air
Funneling back towards the goal when chasing down a defender
Staying compact as a defensive unit with your teammates
Defending as a team - pressure the opponent when you have numbers
Not diving in
Waiting for support before you go to win the ball

It's also important to figure out what works best for your team - Zonal or man marking

Zonal marking gives the team the ability to adapt and also adjust easily with more ease than the stricter man marking system. It's okay to leave the man you're marking in this system, because you have enough time to get back if a long ball is sent wide - you can read the plays and getting a better reliability in getting the ball in time.