If you live where the water runs uphill then start at the top. LOL There is a web site that explains basic roofing but I can't think of it. Maybe at diy.com.
You need to get some help like the others said. There is a technique for starting in the middle such as continuing a row onto the other side of a dormer but you need to be experienced enough to know how to get the lower shingles to match and look right.