In this video, a reply to djjosh, I go over just some of the many things to make you less worried about doing a wedding. Especially for those that don’t have a lot of experience.
Other than the tips on the video, please READ all you can get your hands on about MCing a wedding (the DJ part should be the bit we can all do!).
There are a ton of books and forums out there with advice, so use it all!