You know, they make this liquid that's a bright green color you put in your radiator to keep the water from freezing.

That way, your radiator won't bust when the temperature gets below freezing. It might be a good idea to learn about anti-freeze.
I'll quit being a smart ass now.
Call around to some autoshops like O'Reilly etc... and ask them how to fix it. You might be able to use some sort of sealant and not have to replace the radiator if the crack isn't too big.
Prestone is your friend in the Winter.
http://www.prestone.com/products/antifreezeCoolant.php