How to create a file and append data from html page ?
Short answer: 
you can't. The web browser security and sandbox'ing prevents this.
Longer answer: 
setup a small LAMP stack, Ruby on rails(local host server), Python/Django(local host server) to host your HTML/web form. The local daemon can handle appending data to a file, as you enter it into a form. 
With HTML5, you get special hooks that may allow you to write to the local filesystem, but those may be browser specific and may break from time to time.
I have an html file on my desktop that takes some input. How would I go about writing that input into a file onto my computer? Would I have to use another language to do it (i.e python or javascript?) and how would I go about doing this? On a related note, is there any way I can have javascript start an application from within an html file (the goal is to write to a file on my computer?