There is a lot of scripts out there on the net that have been written already to accomplish this task (www.hotscripts.com example site), but if you don't really care much for the php code then I would suggest using phpnuke or xoops (I hear they are very easy to use).
The easiest thing to do (if you like to code) is to write a php form script that queries a database and verifies the username is not already taken, which if it is good it will upload it. First you must verify what version of php you have on the server since the newest books are php 5 (object based) and most servers are still using php 4.
If you would like a good book to learn this stuff I would suggest
"PHP and MYSQL Web Development" second edition (php 4) by Luke Welling.
If you can't find any scripts for this task let me know and I will look through my computer to see if I can find those scripts.
Best Regards,