Don't post anything on a a social networking site which gives your real name, address, school, phone number or which will allow a stranger to contact you in real life. Make sure you don't ID your friends either.
Don't upload anything that might might embarrass you at a later date. You might not realise it but things you post on the internet now could come back to cause problems for you later on, for it to be shown to the world then do not hit send. Remember once you send it you have lost control over of that lost control of that image or comment.
Never be pressured into taking pictures of yourself that you wouldn't want other people to see. Trust your gut instinct over this. Once again you hit send, you have lost control over that picture and this can cause anxiety and stress.
If you're using a shared computer at school, in an internet café or library then you'll stay logged on even when you close the browser. So don't forget to log off when you've finished the session. Read more tips about staying safe online.