I've found Createspace is quite easy to use.
Unless you choose an add-on such as editing, design or marketing, there is no cost associated with publishing a book, although you will probably want to order a proof and thoroughly check it before publishing (cost around $10 including shipping).
Choosing a size for your book
What you will need:
- Manuscript formatted (page size) to the size of the book. I usually publish trade paperback size 5.5" x 8.5".
- Cover picture (no need to design the cover, you can use the design wizard and use one of Createspace's templates)
- Blurb (about the book) for the back cover. Copy and paste the blurb into the appropriate box when you open design wizard.
- Author picture (optional)
- ISBN (International Standard Book Number) You can request a free number.
- Biography (optional)
A Quick Guide Through the Publishing Maze has more information about publishing and marketing