Hello Doris, different social media platforms support advertising in different ways and have different methods of targeting audiences as well. I would recommend having a look on the help sites for the platforms you are most interested in and they should have plenty of information. I've linked a few helpful pages below to get you started:

Facebook - https://www.facebook.com/business/overview

YouTube - https://support.google.com/youtube/topic/3030592

Instagram - https://help.instagram.com/1554245014870700/