I remember my first game, I was horrible
But here are a few pointers.
Take it slow, don't rush, make sure you blow your whistle hard, and stand still as you make your calls using the proper signals and mechanics. In my first games they really didn't put as much focus on the calls I made, but how I made them. One evaluator tells every new official after he watches them, "Make sure you
blow the whistle, use your voice and hustle" Those are the basics, don't be afraid to make the calls. Good Luck