Choosing a duck call or calls can be difficult for the beginner. When I started duck hunting I had no idea on what call to start with. I went in to a local outdoors store and looked around for a bit and ended up asking someone working there what I should start with. This is always a good idea because most outdoors stores have qualified people to answer questions such as this.
I suggest that a beginner should start out with a poly double reed. They are much easier to blow than a single reed, and reproduce real duck sounds. I also suggest buying a "how to" DVD on duck calling. This way you can watch what a pro is doing and replicate it. These videos show you which calling sequences to use for different situations. Pay attention to this part and use it in a real situation in the field.
Once you have your call picked out and an idea of what to do, just keep on practicing. I blow my calls in my house, out in the yard (some neighbors might not like this one), in my truck, really just anywhere and anytime. Once you get down the right sound just keep on it. Practice, practice, and practice some more.
All of this practice will pay off in the field. There is just something amazing about tricking a real duck in to thinking you are one of them. You will never forget the first time a duck or flock of ducks are flying by your spread. You grab your call, and give them a few quacks which turns them and sucks them in to your decoys. It's a very unique feeling that brings a hunter closer to Mother Nature, in my eyes.