Make sure that you get a copy of the USSF's Advice to Referees on the Laws of the Game. It is really the how to book of soccer reffing in this country.
A good companion is their Guide to Procedures. It covers all the mechanics and where you should be on the field.
Also, since you have been away from the game for 20 years, you may want to check out Strength Training for Soccer Referees. It discusses stretching and exercises to shape up your muscles and overall physical fitness.
Best of luck.
