What should a babysitting ad have in it?

You are too young to place an ad and attract undesirable weirdos. Better would be personal contact with neighbors, relatives, friends' parents, people at church, school faculty and staff. And always you need your parents' permission to work in someone's home.

You can not place any add without your parents anyway, so dicuss this with them. Your mom or dad would have to arrange for it all to happen anyway.