my advice would be:
Think carefully about the info you give out about yourself and restrict what people can read until they are your friends. I've heard that potential employers are searching Facebook to see what they can find out about job candidates, for instance.
Decide on some critera about who to add... I very rarely add anyone I haven't met in real-life for instance. I use Facebook in a totally different way to how I use other sites, mostly because I use my REAL NAME on there.
Don't use Facebook to wash your dirty linen in public. If you fall out with someone just delete them from your list... Slagging them off on Facebook is so
tacky.
Never feel pressured by a Friend Request, you're not obliged to add someone if you don't want to and if you do it will spoil your fun. Just hit "Ignore" and forget it.
If you have a job, don't discuss it. My boyfriend was sacked from his first job, partly because of things he wrote in an online journal. He hadn't used his real name, OR his employers' name, but he HAD put up pictures of himself... And at a tribunal they said his employers could be identified via people seeing him at work. He lost his case.
Oh dear, I sound all doom and gloom but Facebook CAN be fun... Just be careful out there!