Understanding Screen Readers
Learn about the Basics of a Screen Reader
Hear the Difference Between Inaccessible & Accessible Document Content, Courtesy of Normandale Community College
Instructions for NVDA Download
Scroll to the bottom of the page, and choose to "Skip donation this time"
The download will start and you'll see a pop up. Select that you agree, then click on "Create Portable Copy" or "Continue Running" (if that's an option). If you choose to keep it running, you will get a series of prompts that you need to answer - they're all pretty self-explanatory.
The NVDA Screen Reader will then start talking to you, so it's important to know that the way to make it stop, pause, and resume. Take a look at the commands that do those on this Quick Key Commands page. You can also see how the commands work by watching this video - it's a pretty decent one as far as informational videos go.