How do I make a guestbook then?

Depends how complicated you want it to be

one MySQL table storing entries and a simple php script to put in new entries would do the job

Or... a text file to hold entries with a field delimiter and a record delimiter then PHP to parse the text file into a page, and to create new entries.

Either way is a piece of pee...

But the mysql one although sounding more complicated, will be the easiest, as with the delimiter one you need to use regular expressions which can be a ball ache, to filter out the delimiter chars from any text that gets inputted.