If you shared something you shouldn’t have, the first step is to ask the people you sent it to not to pass it on. If someone else posted something you sent them, start by asking them to take it down. It’s actually pretty effective most of the time. Remember not to do anything while you’re mad: give yourself time to cool down and, if you can, talk to the person offline.
If they refuse to take it down, don’t try to get back at them by sharing private things they sent you, harassing them or getting your friends to gang up on them. For one thing, this almost always makes things worse. For another, the more you get back at them, the more it might look like it’s just as much your fault as theirs.
If you’re tagged in a photo that you don’t like, remember that a lot of photosharing and social networking sites may let you take your name off any pictures you’ve been tagged in. On Facebook, you can also select to review posts you are tagged in before they post to your timeline under your privacy settings: facebook.com/privacy.
If you’re on Facebook and don’t feel comfortable confronting someone yourself, or don’t quite know what to say, Facebook has a social reporting tool with some messages you can use and ways to get a parent, teacher or trusted friend to help you out.
For more serious things, for instance, if it’s a partly or fully nude picture or video, if it’s defamatory (it’s not true and hurts your reputation) or if it’s being used to harass or bully you, you can ask the site or service that was used to share it to take it down. In those cases, you can report it to the police too.
If you are in a situation where a person is threatening to share a nude photo of you unless you provide more nude photos – you should involve a trusted adult and contact the police right away. This is unacceptable behavior and in many countries it is illegal.
Remember that you are not alone – you can always talk to your parents, a teacher or counselor, another adult you trust, or a help-line to get advice and support.