Do you read books? Do you have opinions about the books you read?
If yes, then grats, you have the experience to be a beta! The art of beta-ing is the art of figuring out how to explain what you like and don't like about a story and, if possible, explaining how the author could fix the problems you see. (Sometimes the author or other betas don't agree, but that's why we say the business is subjective...) If you have an eye for typos and such, that's even better, but not required.
If you feel honest enough and patient enough to beta, then go ahead and offer. Even if you mention you're not used to it, there will be people hoping to get your opinion on their stories.