Fred - some tips on the defending:
It is hard, nothing like we have seen from PES, but I'm already getting more used to it and I hated it at first.
Close them down:- When the opponent has the ball make sure you are closing them down at all times, hold the run button (shoulder) and pass button (A on Xbox) together to do this. This will make them rush more and possibly let you take the ball away from them. This is also one of the few ways to give players a nudge when they are through on goal. It doesn't always work as through balls are very dangerous to defend on PES 2012, so you might have to resort to a risky sliding tackle or make the keeper run out.
Use other defenders:- If you hold the shoot button (X on Xbox) as well as the run button and pass button it will bring another defender to your aid, this risks leaving another player open of course, but can often work to steal the ball.
One on one:- If you find yourself isolated hold the right trigger and the pass button, this will jockey the man. Then when you find an opportunity to take the ball, release the pass button and press it again with the direction that you wish to make the interception. FIFA has something similar, although defenders dangle their legs a bit too much and it can cause many trip fouls.
The pressing defense is by far the best tactic. You could also field a deeper defensive line if you want to counter, but of course this puts you at pressure from long shots, and on Professional level and above I've had a few players smash shots past me.