How to make a chatbox?

Answer 1

PHP - Will handle all the server side stuff
Writing new messages to the text file
Reading out new messages from the text file
Retrieving the "state" of the text file
Basic security. jQuery/JavaScript - Will handle the client side stuff. This is an AJAX-y application, meaning that messages pop onto the screen (both yours and others) without needing any page refresh.
Periodically asking the server if there are new messages that have been posted
Appending new messages to the chat
Scrolling the chat down to the most recent messages
Asking and setting the user name
Limiting the text input to prevent gigantic ridiculous messagesx
Basic security. Text File - Stores the chat. This is just a simple chat box

Answer 2

just Google "chat box plugin"