For those who are wondering what a beta reader is, it's basically an editor (in the most basic sense of the word). They read over your drafted story, and can make suggestions about possible changes to characterisation, plot, grammar, or any number of other things.
Before posting any stories, I strongly recommend sending your work to a beta reader. If not one of the volunteers listed here, then someone else. It's also usually a good idea to have at least 2 betas - I generally use at least 3.
If you'd like to be a beta listed here, then please fill in the form on the appropriate page for that fandom.