PICK A NEUTRAL SCREEN NAME
Avoid names that include things like baby, hot, sexy, or anything that might give the impression you are someone who is interested in talking about sex.
ALWAYS CHECK YOUR PRIVACY SETTINGS ON EVERY SITE
Make sure your privacy settings are set to friends only. Unfortunately, most sites have the default set to LEAST privacy, leaving it up to you to make the change if you want greater safety. Here's some good info on Facebook privacy settings
KEEP PERSONAL INFORMATION OFFLINE
Don't post your cell phone number, your address, your school or photographs which reveal information (sports teams, school logo shirts, license plates, signs, etc)
THINK BEFORE YOU POST
What you put on online is forever. Those pics of you drunk with your friends at a party? The flirty, sexy shots you took in your bedroom? What seems totally fun now might not be quite so cool when you're applying to college or interviewing for a job in the future.
BE CAREFUL WHO YOU FRIEND
That drop dead gorgeous 15 year-old guy with Abercrombie model abs you're chatting with? He might well be a 40 year-old with a beer gut. Unless you know someone in real life, you have no idea if the person you think you're talking to is really that person.
BROADCASTING YOUR WHEREABOUTS WITH FACEBOOK PLACES OR FOURSQUARE ISN'T SMART PRACTICE
Sure it helps your friends know where you are, but it also gives that information to everyone else. They don't call these the "stalker apps" for nothing.
DON'T RESPOND TO THREATENING OR BULLYING EMAILS OR IMS
Print them out or take a screen shot and tell to your parent or guardian. Report any comments that violate the site's terms of service to that networking site.
BEWARE OF ANYONE YOU DON'T KNOW WHO ASKS YOU TO MEET THEM SOMEWHERE OFFLINE
If that person is an adult, they can be charged with a criminal offense called Online Enticement.
REMEMBER: EVERYTHING YOU DO ONLINE HAS IRL CONSEQUENCES