Aponi I've spoken to a couple of novelists and it's very rare to start at the beginning. They tend to start at the bit that is clearest in their mind. One of them even writes on index cards, then tiles them out all over a wall in her study, and rearranges them. That way she can change the order things happen in the book.

Just bear in mind that things tend to get re-written quite markedly before they make it to publication.