I've started writing my family history, like Berlin Bob I have a website that contains much information which will eventually be in the book, and I have the John Titford book mentioned by Teaurn, it really is good.
I agree with most of the posts in this thread:
1. Make a start, even though your research isn't finished (it never will be, and you could find yourself waiting forever for that last piece of information).
2. Don't second guess your ancestor's motivation. By all means include commentary, but make it clear what is your opinion and what is fact.
3. Give your sources for everything.
4. It may be more manageable to start off just writing a few pages about one person in your family. When that's finished, choose another person and write about them and so on. Pretty soon it'll build up into a book without too much trouble and you just need to decide how the individual pieces need linking together.
5. Definitely use a word processor e.g. Word, or for a great free alternative go to
www.openoffice.org - it makes changing what you've written much easier to alter.
6. If I'm sending someone a copy tend to send them a pdf file rather than a word document - openoffice has a pdf creator built in or try cutepdf. My reasoning for this is that it preserves my formatting and makes it harder for someone to alter (although not much!)
Good luck and go for it.