Best things you can start doing now is developing good habits.
1) Hustle
2) Know your coverage areas and responsibilities as lead or trail and work on those
3) GOOD MECHANICS
4) Remember that sometimes the competitive juices get flowing and players and coaches sometimes get ticked in the heat of the moment so don't have too quick of a trigger in the rec leagues and such. I had a hair trigger when I started at 18. On the other hand, don't let anyone undermine you or do/say anything that crosses the line.
Just work hard, study whenever possible (i keep a set of books in the bathroom
), have thick skin, a patient whistle and a short memory if you mess up a call. Just move on and learn from your mistakes. Oh and one more thing, if you see something and you're not 100% sure of the rule, don't make the call.