Wedding- what to wear?

So my boyfriend is making me go to his cousin's wedding at the end of June but insists that I don't need to buy anything to wear and I can just wear whatever. I've been freaking out. His family seems really conservative and it looks to be a pretty nice wedding. Not exactly just pull something out of your closet material. So I think something new is in order. I just don't know what. My plan so far is to download some wedding shows off itunes and check and see what everyone else wears, couldn't hurt right?