I can think of at least a dozen different ways a car's starter can be wired. Repost with year, make, model and engine and with that info we can give you answer answer that pertains to your particular engine.
check the starter relay circuit and the ignition switch also check the key they can co out of sync with the security system, even having a key from a different vehicle on the key ring can stop a car/truck from starting
does it crank? if not check cables at all ends for corrosion and tightness. does starter click? we may have dodgy solenoid or starter motor. try jump starting.
if it cranks but won't fire? check for fuel supply? spark at plugs?
