First of all Congrats!
Personally, I believe the most important thing a caddie can do...is to memorize the yardages.
Keep a little steno pad....or some type of small booklet that you carry with you....and as you become familiar with the course...mark out the yardages hole by hole.
Know the yardage to the hazards on the course. Know which side of the fairway to "miss" on.
Lastly, I would also memorize the layout of the greens. Know the which way the green breaks etc.
If you carry the clubs, and are able to give the golfers correct yardages etc, and then if asked how the green breaks etc...be able to give a smart, knowledgeable answer. If you know these to items...You will go far.
Good luck! Man......I'd love to play Medinah!!!