Don't do it! Photographing a wedding carries significant responsibility even when the couple concerned are friends or members of your family. If you have not done it before, hire a professional photographer and ask him or her to allow you to act as the second photographer. Playing second fiddle is a lot easier than having to direct the whole operation, and also carries less responsibility. Get some experience before undertaking a wedding alone.
Ideally, a photographer should have some appropriate qualification or distinction, the achievement of which involved photographing people. Alternatively, attend a course on wedding photography run by a responsible professional body. Even then, it is essential to get some experience before handling a wedding as the sole photographer.