How do you set up a administrator account.i seem to have lost mine?

when box where you sign in comes up there is nothing in sign list and i have to sign in every time i can go in control panel and make a account for a guest. i have try ed making one for my granddaughter but tho she in on start up can not get her on sign in list as it ask for her email [i have put my in / and her dads but it will not take it help