Welcome to MobileRead LibraryGoddess.
As AnemicOak pointed out, the Wiki is a great place to start. You list that you already have a Sony 505 so it would seem that your next steps would be adding material that you want to your collection. For classics and creative commons books, a lot can be found in the MobileRead free ebook section. (Just click on the E-Books part of the white on blue menu above.)
Feedbooks offers a wonderful selection of of free material crafted into PDF files for the Sony. If you are looking for Science Fiction then try Baen Free Library.
For purchased sources beyond the Sony store, I have found a lot at
Fictionwise in their "multiformat" offerings. Some people pick up books in the MS Reader (AKA LIT format) and convert it to LRF for the Sony Reader.
Good luck in your travels.