I feel like that title is a bit of an oxymoron.
If you have a Facebook account, you probably just accepted the default security settings during the process of opening the account.
That’s not good enough.
You really need to learn how to take control of your account and understand what information is exposed. I am sure that reading the privacy information on Facebook does not sound like your idea of a good time, but until you are familiar with it, you shouldn’t post anything in your account. (To get to their privacy information, go to “Account” in the upper right-hand corner of your Facebook page. Choose “Help Center” from the drop-down menu. In the left column, choose “Browse Help Topics”. Take a look at the pages dealing with Privacy.)
For a helpful overview on keeping your Facebook information private – including how to display different levels of information to different friends – please head on over to Lifehacker to read this post: http://lifehacker.com/5808175/how-to-keep-people-from-seeing-your-facebook-info.