Hello and welcome. While you're picking up free books, take time to tell us about the kinds of books that you like. In addition to the places list in Daffy's post, you can find free pulp fiction at Munsey's (
www.munsey.com) and fantasy and science fiction at Baen (
www.baen.com).
For purchasing books, of course you know you can get them from Amazon.com, but you can also buy DRM-free MobiPocket books ready to read on your Kindle from Fictionwise (
www.fictionwise.com) and Baen/Webscriptions (
www.webscription.net).
Anything more than those would probably require you to learn more about your computer and the format-shifting programs.
Welcome. Welcome, indeed!