Denu's advice is as sensible and helpful as ever. I'd just second this: many people, particularly women, feel they should go in as subs because they don't know where to start. Don't be surprised or alarmed if you find, when you have some more experience, that you'd rather be on the other end of the leash.
As for boundaries, if you Google "BDSM checklist" you'll find lists of just about everything anyone has ever done or fantasised about. You can either use them to write your own list of Yesses and Nos, or if you want to be really thorough, copy one of the lists and fill in its categories (from "No way!" through "might try it..." to "Oh yeah!") and present it to possible contacts.
And feel free to share any experiences that worry you or make you want advice. This is a safe space.