Caddying is more than simply lugging clubs around a golf course. At a professional level, caddies are considered part of a two person team--golfer and caddie. Learn the basics about caddying. It's a great summer job, a way to be around "the game," and make contacts that may serve for a lifetime.
Dress neatly and have a reasonable haircut. Golf is a rather conservative sport--at least in the venues that employ caddies. You may have a caddie uniform to wear. White coveralls or a white golf shirt and tan khaki pants or shorts will always serve you well.
Maintain a polite demeanor. The use of "Yes Sir" or "No Ma'am" has a surprising ability to impress. Speak only when spoken to. Obviously this may change over time should you caddy for a golfer on a number of occasions, but initially, silence is golden. Golfers are easily distracted.
Give advice when asked and if you know the answer. If you know how the greens break and a player asks you for advice, offer it. If you don't know, apologize and admit to ignorance. A golfer will respect an honest "I don't know." Even if a golfer ASKS, clam up when asked to critique his or her swing. Leave that to the course pro.
Stay up with your golfer. When you reach the ball, set the bag down about four feet to the ball's right (for a right-handed golfer.) Hold it upright so the golfer may choose his club, and after the club has been chosen move the bag back a few paces to the side and a few paces back so you are not in the golfer's peripheral vision. As the golfer prepares for his shot, rest and catch your breath. Stand still.
Pay attention to every shot your golfer makes. A lost ball is a triple bogie for a caddie (even if his golfer only makes a double bogie.) On blind holes, tell your golfer you are going ahead to "forecaddy." Quickly move to a point where you can see his or her tee shot land. To do this, you will need to be in a position like the crest of a hill where you can see the initial flight of the ball as well as its landing area. A good caddy knows his or her course well. After positioning the place where ALL of the shots of the foursome have landed (if you're caddying for a member of a foursome,) move to where your player's ball lies and set up for his next shot. This is another time to rest.
Learn the proper way to hold a pin. Basically, you stand to the side of the cup with the pin still in the cup but not all the way to its bottom. You want to be able to easily remove the pin once your golfer has hit the putt. Make sure you are standing such that your shadow does not fall in the putter's line (path of the ball to the hole). If there is a question of whether or not a golfer wants the pin in the cup while putting, ASK. Simply say, "Would you like the pin, Ma'am (Sir)?" Some golfers use the pin for any putt of 20 feet or more. ALWAYS remove the pin as soon as a putt is struck.
Have a sand rake in your hand if your player hits a shot into a bunker. As soon as he exits the bunker, rake the trap. Learn how to rake traps from another caddy or a caddy-master (an older caddy who runs a caddy program). If your golfer is in a greenside bunker and will probably hit a shot into the green, have his putter ready so if his shot comes to rest on the green, you can simply hand him the putter. He will then be ready to putt while you rake the trap.
Stay out of a putter's line. No one wants to putt over a footprint left by a caddie. This can be tricky if there are four balls on a green, all on different sides of the cup, because you don't want to step on any of the other golfer's lines either. Clean your golfer's ball while he or she is waiting to putt. A towel that is wet at one end is a caddie's best tool. After a club is used, clean its club face as well.