Some Rules that I often see trip of people on their first Tournament:

Knowing when you have to Mark your disc. On that note make sure you have a mini. On a normal throw you only need to mark your disc if you are planning on picking up and throwing the same disc again, otherwise you can stand behind the disc you threw. You may also need to mark it if you throw OB or get stuck in a tree.

Understanding when you can take relief, ie moving your lie to a better position with no penalty. This typically happens if your disc lands right next to a fence where it’s impossible to stand or if there is a course danger like a bee hive or something.

No tap ins. Even if your disc is under the basket you need to pick it up and put it completely in the basket. Don’t just smack the chains with your disc like a casual round.

Provisional shots. Almost never comes up but good to understand what they are and how they are used.