In this quick Arduino tutorial I will explain how you can control a relay using the Arduino Board, one 1K and one 10K resistors, 1 BC547 transistor, one 6V or 12V relay, one 1N4007 diode and a 12V fan ...

Or just use a MOSFET if the relay isn't very fussy. Or, use a combination of the Arduino driving something like the above timer relay, which itself will pull the 12V relay high if that's all it needs.