The functionality of this is simple. The greatest issue is security, and you really have to take that seriously because a file uploader is like an invitiation to a party to a hacker.
Since from previous posts I see that you are using DW server behaviors, not writing your own code, I think you should not attempt a file uploader until you are ready to write and completely understand your code.
PHP SOLUTIONS by David Powers has one or two chapters that discuss this process and adds security for it.
Zend_File_Transfer_Adapter_Http is a PHP class that handles security around file uploads, but this is advanced.