If you want everything you say on Facebook to be visible to the world, that is okay. If you want to set limits on who can see how much, then you need to modify your privacy settings. Best explanation I have seen on how to manage your privacy is found in this article from the Christian Computing Magazine. Took me a while to wander through the settings and instructions at Facebook to learn what is covered here in just a few pages.
By the way, the magazine is free. You can read it online or get email delivery. Check it out. Sign up for a free subscription.