Most dogs will automatically guard and protect their owners, so please don't try and train a dog to be a guard dog. Only professionals should do this, and even then this is not always a good idea.
That said, there are certain breeds which tend to have more of a guarding instinct: German Shepherds, Akitas, Dobermans, Rotweillers, to name of few of the more popular breeds.
I recommend researching each of these breeds to find the perfect fit for your lifestyle and personality.
There are two books I recommend and keep on my own bookshelf for reference:
1. Tortora, Daniel F. The Right Dog for You: Choosing a Breed That Matches Your Personality, Family and Lifestyle. New York: Simon and Schuster, 1980.
2. Walkowicz, Chris. The Perfect Match: a Buyer's Guide to Dogs. New York: Wiley Publishing, Inc., 1996.
To raise a good pet, it is important to socialize the puppy from the time you get it at about 8 weeks (no earlier - not good) until the critical socialization period ends at 3-4 months, and continue to socialize it and train it using positive reinforcement training for the rest of its life. To get a great relationship with your dog and create a reliable partner, use clicker training! If you use any kind of force-based training (shock, choke chain collars, prong collars, leash corrections, dominance) you can very easily create an aggressive dog!
Maybe you don't even want to deal with a puppy - it's a lot of work! You can find great dogs through breed rescues and in animal shelters that are there for no other reason than they do not have a home! If not rescued, these dogs will be killed. Many, many great dogs are killed (millions) every year due to a lack of a home.