Creating a chat room

i have made a website (using a website to do it) and i want to add a chat room to it
you can copy codes and stuff like that form other places and theres some free chat gadgets on the site but i want to create my own!

on my site i have admin, moderators and normal users, i want to add priveleges to certain groups (like admin and mods) (people have to log in to use my site)
i have no idea where to start!
i havent a clue about most of this so please can sopmeone help?

thanx