Adding Text to a .txt file

a hi hi, i was wondering if you could assist me in how to make a html form with like an input box and a button and u put some text in the box, and send it, and it will end up in a txt file…?

abit like a guestbook but with it goign into a .txt file… i would appriciate it alot of you guys could help poor old me… :smiley:

thx in advanced