Hi!
You should not power the pump directly from the Arduino pins. To much current, it can damage the Arduino.
Buy a relay module:
http://www.buyincoins.com/new_en/details/new-one-1-channel-isolated-5v-relay-module-coupling-for-arduino-pic-avr-dsp-arm-product-9098.html
You connect the digital output directly to the module and the pump to the relay side of the module.
Alternative you can build it yourself. It is just a transistor and a relay with a freewheel diode.