My rule of thumb is this: Don’t share anything over the internet which you would be ashamed of if someone saw.
Be wary of sending explicit images over messaging as well as harmful information (gossip and hate). The former has legal implications and the latter is a social issue. There are numerous laws which restrict the transmission of explicit images to others, especially minors. If caught, serious charges could be brought against you. If you believe you can send explicit information without consequences, please double check before you do. On the topic of gossip and hate, I recommend double checking what you send for characteristics such as tone. I am no expert on cyber bullying, but I recommend watching what you send anyways.