Posting an ad online, even offering to be a babysitter, is dangerous due to the many scammers, perverts, and predators. Posting an advertisement for yourself in the newspaper, on a bulletin board at a community center, etc. is dangerous.
You can still place ads online for babysitting jobs and offer your services via job portals, bulletin boards, and web portals. Be careful, especially if you're under 18. Before you meet potential clients for your babysitter job, make sure to have a parent with you. You can search babysitting jobs at http://swishboom.com/.
Clients will appreciate your thoroughness and that you have a parent to help you if things get scary. It is a good idea to schedule more than one meeting with clients before you accept the job. One to meet the parents, one to meet their children. You and your family will have more time to get acquainted in a safe and friendly environment.
Find a job that allows you to work with children on a regular basis. Volunteer in the children's area at the library or at a local center. Parents will feel more comfortable when they see your resume at the child care centers. Volunteering at after-school or before-school programs is also possible.
You can also contact teachers from your local elementary schools or colleges for great resources. Ask teachers if they would mind recommending you to a few potential babysitter jobs to help you reach more parents. You might also consider letting your parents know that you are seeking babysitting work.