Stan, good luck - and just go out to enjoy...
I would say though get in to good habits from the outset. As
@heedmatt said, get there early - it sets the tone. When walking the pitch, keep an eye out for dog mess and other hazardous items (and remember, it is not your job to remove them - let the home manager know and it is his responsibility to clear). Once the nets are up, check that you are happy with them - ie fastened and pegged down well. Check the goal posts. Get the match balls and check them. It's U16 girls, so there will be jewellery - make sure the girls have removed earrings etc when you do the boot check.
My routine then is 5 minutes before kick off get captains, managers and whoever is running the lines to the centre circle, do the coin toss and brief your CAR's as to what you want from them. In some areas CAR's do offsides, in other areas they don't - check with a colleague beforehand: I always ask CAR's to wait for an offside player to touch the ball before raising their flag, unless its one on one with the goalkeeper and its clear another attacking player will not get to the ball. Tell them now that if you have a different view to an offside, you'll indicate to them to lower their flag and carry on (but also thank them during the game if they are getting things right - a thumbs up or mouthing "good call" to them when appropriate). Doing this 'briefing' in the middle means you are only having to do it the once, and also means everybody hears the identical message.
Then get the teams set and off you go! Don't forget to start your watch(es)... (I always start my watches before blowing for KO, to ensure I then don't forget).
Have you practised with your whistle yet? If not, and whilst it might sound daft, have a practice at home - you don't want a barely audible wet whistle sound when you blow during the game. During the game, use the whistle differently depending upon what you are blowing for - careless foul: short and sharp, reckless foul longer and harsher and so on.
Finally, you will make mistakes. We all make mistakes. Don't feel disheartened during the game when you recognise an error you have made. Push it to the back of your mind and review them after the game - it will help you improve game by game.
And enjoy! Report back here to let us know how you got on.