Hi ibrahh2005,
that's still not enough information.
Where does the water come from you want to move? In some areas city water supply slowly fills a reservoir (mostly at ground level), which is then pumped into an overhead tank.
You certainly wouldn't like your pump running dry, so this has to be considered when designing a control device.
The pump activation also depends on your desires, meaning do you want to activate the pump if the water level in the overhead tank is not at maximum and there is water in the reservoir, or do you want to activate it if the water level in the overhead tank is down to minimum or 1/2 filled?
Boncuk