Keeping it simple. Get a water tank float switch which is available at many hardware, farm, or marine supply stores hook it up operate your pump motor. Add a relay with appropriate contact value rated for the pump motors voltage and current.
A battery or low voltage transformer would provide power for the driving the relay on and off with float switch's open and closing.
A second float switch could be added to the circuit placed lower in the tank this would allow the upper switch to turn pump off and the lower to turn the pump on. This keeps the pump from cycling every time a small bit of water is used.
Two float switch control.
