TREAT OTHERS THE WAY YOU WANT TO BE TREATED... Don't leave disrespectful comments on someone else's posts. If you wouldn't like it, then that other person would not appreciate it either!
Before you post something, make sure you won't be regretting it in the end. Whether it's a picture, or a comment... Be careful because it might come back to haunt you later in life.
When creating a profile for a social network site, be sure to keep your passwords private. Make sure your personal information/details in your settings are private too!