What kind of signals are you asking about. Block signals along the track? Dwarf signals for things like yards? Grade crossing signals? Train order signals at stations? Interlocking signals where two rail lines cross?
As you can see there are a lot of different types of signals and there are just about as many physically different signals. There's semaphores, searchlight, position, and color signals, just to name a few. How big a layout do you have? What era do you operate in? Do you want the signals to actually do something as trains approach and go by them or are they strictly decorative? Sorry to hit you with so many questions but, like Grandeman wrote, signals are one of the most potentially sophisticated parts of a model railroad.
