Scour yard sales, thrift stores, book sales and your own basement for books to sell.
Look for books that are nonfiction in clean condition. Avoid best sellers, modern paperbacks, cookbooks and diet books as they usually sell for next to nothing. (See notes)
Try to find a niche book market that interests you. When you know approximate values for books in your niche market, you will sometimes be able to find bargains at half.com and ebay and you can make money buying and reselling these books at Amazon for a tidy profit.
Gently clean the book and prepare it for sale. Read my eHow article;
"How to Prepare Your Books for Sale"
http://www.ehow.com/how_4493047_prepare-books-sale.html
Click the "Amazons Seller Quick Start" link in the resource section below. Amazon will give you step by step directions to list your books. Amazon gives clear guidelines on rating your book ie new, like new, very good etc.
When you are listing your book, take a look at other copies of the same book, to see what the sellers are charging. Also look at the sales ranking for the book. If it is not selling well, you may want to price your book at the lowest price point so if a copy sells, it's likely it will be your copy of the book.
Price your book where you can make a profit. Amazon is full of penny sellers. These merchants make their profits through postage. They do mass volume. This only works if you have an enormous inventory and a pro-merchant account.
After your book sells make sure you package it carefully and ship it within Amazon's two day time period. Read my eHow article; "How to Package a Book You Sold" (http://www.ehow.com/how_4498068_package-book-sold.html)