It starts with an input form* of the information you are going to allow the visitors to post. Then comes the database table creation and the script to cleanse and enter the data into the database. Then you need a page to pull the data from the database and display it on the correct page.
Perhaps the most challenging part comes last, an admin section where you can review, approve, reject and/or moderate any posts that were made.
There is a whole lot of thinking and flow charts to be done before the first line of code is written. *Actually asking yourself what information you want to see from each poster is the very first step because the input form including any hidden fields will be determined by the output you want to see.
Cruise some sites and look at their comment forms to get a feel for what you do and don't want. You might also wish to read this thread where I question the common concept of allowing links for posting a comment. http://www.v7n.com/forums/social-net...ml#post1828918
You might be able to find something that is GPL licensed or low cost that you can mod. A guestbook script and a comment form are not worlds apart. A few DB fields + - and they are virtually the same.