Creating a prog to edit files with certin extention

I need some info on how to go about creating a prog to edit files with the .co extintion. I have microsoft visual c++ 6. If someone could tell me the things I need to know about .co file structure(or any file structure) to make a program for the sole perpose of editing this file. I am new to this(like you can't tell) so any links, suggestions, pointers, and instruction would help me greatly. thanks

http://www.wotsit.org

they might have some info...

Why not just load the file directory like normal, then if (strstr(filename, ".co")) { ... }