Ask questions if you're not sure about something. the people in your card will likely be glad to help you out. Better to ask than break a rule you weren't sure about.
Adjust your pace of play. A tourney round plays slower than a casual round. Every player waits for the others to throw, and stays out of their line of sight. Don't hurry, wait until it's time to approach your disc and throw. After a player makes his putt, let them clear the disc out of the basket before you throw.
Have fun! It's still disc golf, but playing in a tourney just feels different, IMO. I played my first tourney a few weeks ago on my home course, and it felt like my first time there. It was a blast!
Be on time... Better yet, be early. There will likely be more out of bounds/mandos than you're used to, and you want to be at the player's meeting to know about it. It's also nice to throw some warmup shots. I tend to goof my first throw alot, and it sucks when it really counts.
Bring plenty of water/snacks. A two hour casual round will take three hours in a tourney. You don't want to be hungry/thirsty.
Most of all, however: Have fun!!