A lot of great stuff has been covered already. So let me add a touch of perspective. Your first game as a ref can be a wild ride. Play seems like it's coming right at you at 90 mph. Don't panic. you'll start to develop the mental filters we all use to reduce all that activity down to the parts that matter. And over time things you'll struggle with at first will become automatic.
As has been mentioned, it's tempting to go out there with 100 things to focus on floating around in your head. That will not work. You need to narrow your focus to a very few things that matter most. Then, game after game, add one or two new things to work on. Good officials became good over time.
If it were me, I'd focus on these three:
1. Be where you are supposed to be and look where you're supposed to look.
2. When you see something wrong, blow the whistle. Hard.
3. Relax and enjoy the experience. During every time out and quarter break, stop, take a deep breath, unclench your shoulders, and smile. Then go talk to your partner.
And report back on how it went.
