There are three main important rules in golf: keep quiet while opponents are hitting, treat the green with care, and rake the bunker.
Golf is a very social sport, and oftentimes you and the other three players in your foursome are going to be chatting throughout the round. While that is perfectly fine, if somebody in your group is about to take his turn swinging, you have to keep quiet. One of the rudest things you can do on the golf course is talk in somebody's backswing.
The green is where you putt and is extremely fragile. When on the green, there are a lot of rules you need to follow. You are not allowed to drive your cart on the green. You need to park your cart on the end of the path and walk toward the green. You also can not place your bag on the green, as you need to place it just outside the green, or leave it in your cart. Another rule is that you can't run on the green, as that can be damaging to the surface. You also need to fix all ballmarks that you might see while your group is there.
Unfortunately, all golfers are going to find themselves in the bunker from time to time. When you are in the bunker you are going to create a lot of sand mounds from your swing and also all of your footprints. It is extremely rude to the people who are using the hole next to not rake. You need to make sure that you leave the bunker the way you found it, because if a player's ball lands on your footprint, he or she is going to have a very tough time getting out of the bunker.