When many post on the internet, they believe they are talking to a group of friends or peers. Sadly, recent news proves this is not always the case.
It's hard to write what you should and shouldn't do on the internet as each site has a different take on things. They have different rules, different amount of publicity and different users.
The main rules for keeping safe is to think before clicking that submit button. Are you prepared for that comment to come back to you? A comment about the company you work for could lead to dismissal if the meaning can be put into a libellious comment.
Advice on the internet should never be taken literally or as a professional opinion. The user that could be pretending to be a helper could be someone who is younger and more immature than you, or worse, has their own agenda.
Personal Details, be careful what you leak on the net. Fraudsters and stalkers could use this information against you.