What kind of relay do I need? I know nothing about electronics but I'm installing these horns myself.

Are you adding to the existing horn system or replacing the existing ones?

I'm going to replace the existing one, so I'll just be using the positive from the old horn as a trigger. The Cadillac horns are self grounding so I just need to be able to connect the wire from the battery and the positive from the horns to the relay as well.