Once you've written and uploaded your book, Lulu will allow the book to be sold on their site. If you added your own ISBN or got a free one from them they will also add your book to Amazon , Barnes and Noble, and other sites around the world. There are alos "paid" services to have them edit, make book cover, or even adverstise your book. These are voluntary but are there if you choose.
In a nutshell. Lulu provide the tools to turn a pre-prepared manuscript in to a book. The page size has to be compliant with what is available here, so it is best to look first.
This should answer all your questions >>
And as previously said there's this also >>