Encoder/Decoder help

I want to write my own encoder and decoder what would be a good way to go about it i know you just use a module but i was wondering how would i change the text like should i add like a number to the character value or something like that could someone help me out?

There is a tutorial on encryption in the tutors corner:
http://www.visualbasicforum.com/t31778.html

thank you J0n