Read ALL of the rules over and over again. You can't apply the rules unless you know them. Know them cold.
Relax, enjoy yourself, and if you have the opportunity to take some plays off (many scrimmages have extra officials there) step back and watch a veteran work some plays. If not, see if you can get someone to film you. Not only can you have someone review it with you to show you where to improve, but years from now, when you're a solid vet, you can look back at how bad you really were.
Ask lots of questions (here is a good start). When you watch a game, watch the officials, not the ball. When you officiate a game, watch the action, not the ball.
Remember that appearance is probably 80% of the job. Dress to perfection. Shoes sparkling, hat crisp, shirt pressed. No stripes showing through your knickers (tuck them into your undergarments). If you step onto the field looking like you are an inferior official, the perception is that you are an inferior official. That goes for weight too. If you're overweight, no matter how hard you try and how much you hustle, you'll be seen as lazy, out of position, etc. Perception is reality. Look the part.