Welcome, Rugbyboot.
As my learned colleagues here said, you will do some mistakes. Take any mistake you make as an opportunity to improve. If you can, have a more experienced ref around, someone you trust and able to give you some feedback after the game. That "extra help" (not for making the calls, that are yours, all yours and only yours) was unvaluable for me: I had a friend, who is also among the "top 20" refs in the union, running as TJ. After the game, around a couple of beers or three, he gave me lots of useful advice and positive criticism.
If the game is recorded (even by any player's parent), try to get a copy of the tape, which is quite easy in these "digital times". Then, back at home, calmly watch the tape, go back and forth around critical moves, and analyze your performance.
Enter the field with the same spirit of joy you had as a player, and repeat for yourself as a mantra "safety first, safety first". Above all, enjoy the game (you've got the best seat available!).
And let us know how it goes!