As defending is now "tactical", "how to defend in Fifa 12" is a bit like asking "how to attack in Fifa 12".
There are various strategies, none of which are the definitive answer.
My personal defensive tactic is as follows;
a) Control the player who is going to "put-off" the attacker. This is the player who will get as close as possible and disrupt the attacker using grab/jostle. I would try to use a defensive midfielder for this if possible.
b) Bring in an additional defender using (xbox RB). This defender will close in and should have an easy job getting the ball from an off-balance attacker (you are unsettling him with your defensive midfielder).
c) A quick switch of players and you should be in control of the tackling player who you brought in with RB, and be in possession of the ball.
It is obviously not always as clear cut as that, and will all happen very fast.
Good Luck.