I went into my first tournament with absolutely no offline practice against anyone other than my friend and the computer on edge master, lol, so the best I can tell you is just that you absolutely must know your character inside and out. Also, know your play-style inside and out. Compare how you fight, to what your character is capable of and what he or she can deal with.
Lets say your offence for instance, having trouble getting your taki in on a kilik user? Go into training mode, set the computer to edge master and find a way to avoid getting poked at from a distance and learn how to move in on a kilik quickly, do your damage and get out of there. A lot of people disagree with fighting the computer, but the computer is almost 90% of the time going to read your controller and GI or avoid most of the things you're trying to do before you actually even know you're going to do it good. In that aspect, if you can do a reasonable amount of damage to the computer against those odds you'll be fine against human opponents.
No defense? Do the same thing, edge master. Practice on guarding against characters you know that you should be wary against.
But always keep in mind whats reality and what is not. Obviously a xianghua player isn't going to out-muscle an astaroth user and trying to do so is asking to get whooped. So, they have to make astaroth play their game, speed and mix-ups. You do the same. Never play the opponents game, ever! Because they will win 99% of the time. Only do so when you feel safe with letting yourself stand right in the middle of their storm and its dire to winning the match. (Daigo vs J.Wong anyone?)
One last thing, GET-ON-YOUTUBE. Nothing better than studying the shit out of tournament matches against well-known, well-respected and strong players. You can learn a lot! I owe my abilities with Setsuka to Cheeseoftheday's videos.
Edit: Don't go in there with a losing attitude. If you "expect to lose" then you may as well stay your ass home. In my own personal experience, absolutely nothing is "impossible" with fighting games. You didn't go to the tournament to waste your time, did you? No, you came to win. Even if you lose, you came there to win.