Defending:
Jockey. Don't rush defenders out, just jockey your man and bring in a 2nd defender if you need to -- again don't abuse this, people will get pulled out of position. Try and predict what they are going to do. People aren't that smart; if someone keeps making a run down the wing to cross, or skills and cuts in, you know it's their preference. Expect it, wait, take the ball.
Attacking:
Crossing is pretty poor, only cross if you've got someone in clear space. 1-2 passes and through balls are one of the best ways to get the ball to a striker's feet in a good position. Having someone good at dribbling is good too; it's quite easy to win pens and FKs, and scoring FKs is very, very easy when you get the hang of it. Finesse shots from close range, tucked under/to the side of the keeper are probably the best way to score. Don't shoot from distance unless you're winning comfortably/desperate/bored -- that said, I've finally got the knack of long shots in this game and they're very satisfying to score!
What kind of problems do you have specifically though? Struggling to score, defend, win headers, do you know how to do basic things like 1-2 passes? Etc. I'm pretty good at the new FIFA, though I mainly play UT where pace is exaggerated. However I don't like the game that much and no matter how good you are, you will lose to crap people playing exploitative tactics, so don't get too concerned about losing!