Go find some youth league/travel league/AAU games to get some actual on the court expierence where you can work on positioning and mechanics and getting the jitters out. I don't care what level you do the first time you roll out there, you will have butterflies and be nervous as heck....My personal advice is not to make your first game with some HS teams.....But I am sure some have done it.....
The lower level stuff will allow you to work on things a little at a time where the games don't mean as much and you can get comfortable with what you are doing...
Stay away from anything rec league younger than 5th grade so you won't be so overwhelmed by the
A-B-C-D-All of the above contact is a foul, which one should I call!? 
Watching other officials at the HS level is good too....