One thing many people can do is to get a dog. Almost any breed of dog can be a good “watchdog”, even breeds that are generally not thought of as good guard or attack dogs, like Cocker Spaniels or poodles. The key is simple: will the dog let you know if a stranger is around? Most all dogs can easily be trained to do this. But you also have to train the dog to stop “alerting” once you know. Again, this is easily done.
A “guard” dog does more than let you know if there are strangers around. They will actually guard you and your family. Breeds of dogs that are generally considered good “guard” dogs are: German Shepherd, Belgian Terveran [sic], Doberman, Rottweiler, Giant Schanuzers and Kuvacsz [sic].
I don’t consider “Attack” dogs like pit bulls good for protection because they can be very volatile, but opinions differ.
The key factor is, If you are going to get a dog, be sure to train it and work with it regularly. Personally, I think Schutzhund training is best for a guard dog but it is expensive and time consuming.
One last thought–If you have children be careful what kind of dog you get and what type of training you give it. And be sure to teach your children how to behave around the dog too.