Make sure you use strong passwords, use special symbols (EG -) and make it long.
Set security questions to which only you would know the answers to.
Be sure that you don't reveal where you live, where you go to school or work and financial details.
If you get a friend request from someone who you don't know, don't accept it as it could be someone out to harm whoever they can harm and also can be a fake account.
Be cautious when you click website links and always make sure they are safe, safe links will always start with http(S).