You're in for some fun. MobileRead is a great community made up of fans of electronic books and the devices upon which they are read. Be sure and hang around and tell us what you think of the Kindle you've ordered and what you're reading.
As for getting reading material, you need to be aware that there are sources other than Amazon -- though of course, you'll shop at Amazon to get your fix of recent publications.
However, first peruse the available books uploaded here to MobileRead. Anything in Mobipocket format can be read on your Kindle. You can jump straight to the book listings at
https://www.mobileread.com/forums/ebooks.php or click the "E-books" link in the blue menu bar under the logo.
Then, if you didn't find enough books to read, amble on over to Feedbooks (
http://www.feedbooks.com) and ManyBooks (
http://manybooks.net) and grab even more free books. If you're a fan of pulp fiction, one of the best sources is Munsey's at
http://www.blackmask.com/
Oh yes, I forgot to mention two sources of excellent science fiction. First, Baen offers many of their titles for free at
http://www.baen.com -- and you can buy even more Baen titles at WebScription eBooks (
http://www.webscription.net). Steve Jordan is an SF author who frequents these forums. He offers one of his titles for free at his website --
http://www.stevejordanbooks.com/ -- and an amazingly low-priced bundle of the rest of his novels.
From time to time, you'll find postings about special deals and free books to be found elsewhere -- as I said, keep on hanging around here!
Welcome -- Welcome, indeed. Hope you enjoy your Kindle.