Don’t share anything unless you are sure you want everyone to see it.
Do read all the privacy settings. Do click on the “See what your profile looks to other people” link.
Do make your profile unsearchable via Google. Do decide whether or not to make your profile searchable among all Facebook users, as well as who can message you.
Do realize that although every photograph and album has different privacy settings, there are a couple of things you can’t control: 1) all cover photos and profile pictures are public. If you replace your cover photo or profile pic, remember to make the old one more private (set to “friends” instead of “public”). 2) If one of your friends tags one of your photos, it becomes visible to all of their friends. Actually, that last one applies to any kind of post, not just photos.
Do realize that every time you like or comment on a post, that event shows up in all your friends’ newsfeeds, even if they are not friends of the people whose posts you are liking or commenting on. You have an audience every time you like or comment.
Do realize that if you use a fake name, Facebook may catch you at it (this actually happened to me) and threaten to lock you out of your account. You will then have the option of either changing your username or providing identification to prove that the fake name is real. If you screw it up, you’ll lose access to everything you’ve posted on Facebook, as well as your contacts. Further to this point, given that access can somewhat arbitrarily be taken away, if there’s anyone among your Facebook friends who you need to be in contact with, make sure you have a backup way to communicate with them.
Don’t take quizzes or answer surveys that require installing an app or clicking an OK button to proceed. More often than not, they’re taking your private information, and they may actually be hacking your account.
Don’t give them a phone number or email (other than the one you signed up with). Personally, I have a Facebook-dedicated gmail account that was only used to sign up for my account. It gets ignored, so it doesn’t matter to me whether anyone gets hold of it. You can give them a phony birthdate if you wish (probably keep a record of it in case you’re asked to provide it at some point); you can also hide your birthdate from your profile. I use my real month/date because, heck, birthday messages from friends are nice. I’d recommend telling Facebook you’re younger than you are, because the ads for anyone over 30 are really fucking depressing.
That’s all I can think of for the moment.