More info is needed for an accurate response, speaking generically, you need a database, which will store a username, user ID and a password at minimum (A user ID isn't actually essential, but indexing a text field takes more time than indexing an integer field)
Further than that, depends entirely on what the site is for, and until you have a database structure drawn up, I wouldn't even bother thinking about a management script.
You have asked a very vague question, the best apporach would be to search the internet: link 1link 2, then get a script going and ask specific questions if you encounter problems.
ouch on link 1 and link 2. I guess I just really have no idea where to even begin. I have an idea for a website, and I know how to do a lot of it now, but I don't really know where to start. I'll have to do some research. Thanks for the reply though.