There aren't really websites to make music, there are programs You have to download them, like Garage Band, Acid Music Pro (0.5/0.6). But they're usually quite expensive, Acid Music Pro is around $70 to download. And, as Mightybalto said, you don't have to compose music for a music video ^^

You can make music on jamstudio.com. It is easy to use. However, MightBalto's got a point.

You don't have to use a website, you can use windows movie maker to make a music video

You don't have to make music for a music video. You can use songs that have already been made. Besides, composing music isn't easy.