We really need more information than this. Are you planning on storing this information in a database, flat file, CSV file? You say you're already using a guestbook. Perhaps you should post the code here or a pointer to a webpage that has the code, so we can figure out how it stores data and how the editing needs to be done.
The guestbook stores the entries in a flat-file database; however, it creates just your standard html file..i.e., no delimiters to differentiate between the entries.
Tackyon (see below) summarized it very well as to what I'm looking for.
Here is the guestbook script: