Html Register Page?

I need help with setting up a html register page?
What i want is to have a html register page where the user types in the desired user name and password and then is creates an account for the user. I want their data to be stored in an xml file or other file type which better suits the nature, which can later be retrieved in flash.

Where do i start. Are their any post on this topic that my help or is their another page i can check out to read up on.

Thanks in Advance

Also im goin to have Sevral thousand users register so i was just thinking that using xml is a bad idea because loading the xml file with 100 users details is bad enough so i was thinking msql but im not to sure on how to set it up properly. The web server where i have my things set up lets me do anything on it so theirs ne restrictions their :)

Thanks in Advance