Controlling 12V 12A DC Pump using PIC
Last time I had a similar need I used a transistor and a control signal from MIC. I recomend you to check this link : MOSFET.
It's a very good guide. I used a very similar circuit, however inster of a MOSFET I used a FET transistor.
Here is a simple control circuit: 
Edited: In your case change the square source to PIC output.