It's your first tournament. It's very likely that you'll get destroyed. In your first, and your second, and really in every tournament, play to learn. Optimize for finding problems in your play so that you can practice ironing them out at home and in friendlies.
Your goal should be to think about decision-making mistakes that you're making (things like hitting shields unsafely, trying to hit them after the timing window is over, bad out-of-shield / off the ledge options, etc). Hopefully if you make such a mistake, your opponent will be able to punish you; make a note of what action you took (and what position / state you and he were in) that let him hit you, and try not to make that same mistake again.
Try to play as many friendlies with good players (or at least, players better than you) as possible, and ask about what kinds of unsafe things you're doing. Good players can help you recognize the mistakes you're making faster, both by punishing them and talking about them (if they're willing to.) Take advantage of that!