Great questions. Lots of us have experience taking caddies for golf, lots of us also have experience BEING caddies. I have both. Hopefully shivas chimes in also - his thoughts on caddies have always been right on.
My advice is this: someone once said the only things a caddie must do are "show up, keep up, shut up." That's not all that far from the truth. Be on time, be diligent, officer advice only when asked. If you conquer those, you'll be in the upper echelon of caddies at the club before you know it.
On top of that though, WORK HARD. That is, go beyond what you think you need to do. Rake bunkers for the other players... working with the other caddie(s) in the group to get that done. Forecaddie - never lose a ball. Again, work with the other caddie(s) to get that done. Always have the wet towel ready to clean the ball after it gets to the green. All of that is common sense.
Don't worry about course knowledge - odds are you'll be carrying for people who know the course better than you ever will anyway. Just observe, gain experience, offer such when asked and you will be fine.
Oh, get the distances DOWN. That's one thing you'll want to be sure of. Again, offer only when asked, but when that happens, you need to be right. And be confident when giving that advice. Heck a good yardage guide will get you started... but over time you ought to come to know the course such that you have a great idea of the distance to anything from anything. It will come over time.
That's all I can think of... Hopefully others will offer more. But seriously, the first part is the most important of all. If you are punctual, quick, energetic and nice, you will be sought after for sure.
TH