Cyn_Finnegan

Put what fandom you'd like to beta for
Be sure to state what you will NOT beta, as to story type and pairings
When making your post, show good language usage, so the authors looking for betas have some idea as to your capabilities.
Finally, tell us a little about yourself, if you like.

Those are the basics of what you should post. The rest is up to you.
To expand on the above a little, something else that would be a good idea is to provide a link to a profile where your stories are located, giving people a chance to read your stories to see if your writing style is appropriate for them. Some writing styles just clash too much, and would likely cause arguments over what's "right" and what's "wrong". If you don't have any stories uploaded anywhere but you have them on your hard drive then tell people that you'd be happy to send them some stories of yours for them to go over. If you don't have any stories anywhere, then... why are you offering to become a beta when you have no practical experience? Would you offer to teach someone how to fly a real plane simply because you've read a lot about them? I should certainly hope not! Being an avid reader is no substitute to being a writer. If you're offering your services as a beta then you really should be able to prove that you know what you're talking about.
Finally, if you later change your mind about being a beta reader, or you feel you have enough on your plate without taking on any more, then it would be a good idea to come back here and update things to show that you're not taking on any more work. That way you save everyone's time!