If you decide to become a member then go to your profile, Click the mother slot. Add her first name, middle name (if she has one), and maiden name in the "last name at birth" slot. Then you enter her date of birth, her place of birth, and sources.
If she's married, add her husband's last name below her "last name at birth" slot. Then "add mother" ie. save.
It's not required for her to have married or still be married to your father. You simply leave your father slot empty. You can add a step-father by adding a husband to your mother's profile and, at the very bottom of the page, uncheck him as your birth father.