Qualifications
A deep, thorough knowledge of golf is a good starting point for any caddie. That said, you can also pick it up as you go—so long as the people you're working for know what they're doing. This isn't academia; the only real qualifications are enthusiasm and the ability to be a networking butterfly.
Then again, if you really want to make the pros, you should do plenty of golfing on your own as well. Many pro-caddies are excellent golfers, too. It's simple, really. Learn golf. Live golf. Be golf.