Displaying certain text for guests only

[The Quibbler]

I need to display a message only to guests on the main page of my forum, where registered users would see nothing. I realize this is the simplest of simple things to do, and that it might not exactly belong here as it doesn't require any file edits - but how would it be done?

Which leads me to my other question: how does hacking vbulletin work? I'm fairly familiar with php; I know enough to do some basic stuff, anyway - but I have no idea how I'd use that to modify vbulletin. Are there any online hacking guides you could give me a link to that I might have overlooked in my search?

I know the forum description said "If it's not a release, not an idea and not a cry for help, this is where it should be." - and I know this is a lot more like a cry for help than I'd like it to be - but I couldn't find a better place for it