You will have to start by selecting a water pump. Preferably a DC pump, 5V or 12V, those are the easiest and safest to work with. If you need more pumping, then maybe a 240V AC one.
Based on the power rating of that pump you can design the rest of the circuit. Most DC pumps can be switched easily with a MOSFET. You may need a flyback diode and possibly also a filter capacitor in that case, depending on the pump. High voltage pumps need a relay.