If it's your first tournament, I'm gonna be really blunt and say you're probably not going to do very well, unless you're secretly super ultra spectacular right off the bat. This is how it has been for 99% of the community when they were starting out. Follow suit, don't expect much out of your performance this first time around unless you're really confident in yourself. People simply have had more time competitively with the game than you and will probably be able to take advantage of nicks and holes in your gameplay which can only really be tempered through experience.
And hey, if you don't expect much out of yourself the first time around, it won't sting as much when and if you don't do well. And if you succeed, then it'll feel all the more awesome, right?
First things first, before you even go, make sure you get a reasonable amount of sleep, and eat some breakfast and all that good stuff. It doesn't seem like much but I can tell you from experience that it can make a difference with both your gameplay and your demeanor.
When you go to your tournament, don't go expecting to win. Instead, you should go with intent of having fun, meeting new people, and upgrading your experience with the game. At least a few of these should be on your to-do list:
- Make a good first impression, look and be nice to others, etc... try to understand what your scene is like and who the important people are.
- Play a huge amount of friendly matches with others. Playing against other people who are at a tournament level will hopefully give you some good ideas on how to adjust your own gameplay to the max.
- See if you can seek out someone else who plays your main(Lucas?) and see if you can get some tips and advice.
- If you lose your tournament matches, don't get down! Focus on how you played in those matches, how your opponent played, and how you could improve your plays for next time. Ask for advice after matches!
- Most importantly, have fun! Smash tournaments are a really enjoyable social event generally full of swell people. Make some acquaintances IRL, get connected with Facebook, do all that good stuff! It pays off!
What's most important is making connections and learning more about yourself and the game. Do what you can!
Of course you can disregard all of this if you win your tournament, heh heh...