Changing a password is relatively straightforward. But knowing CPR does not make you a surgeon. Obviously being a doctor is much more involved than being a sys admin (apologies to any MDs reading this), so what I'm trying to say is: this is much more complicated than it looks.

Change the password? Is that a login password, or an email password? Is this a shared-hosting system? How many domains? What brand of Unix are you using? What version? I've been doing this a long time, and even I would need to look up half that stuff you wanted. Everyone here would be happy to help you by answering specific questions, but this is a very broad topic.

Usually, a server is planned based on need and then implemented based on operating procedure. The specifics should be documented when the server is configured. If there's no documented procedures for these common tasks (i.e. billing!?!!) then you're left to reverse-engineer the original setup. And that's a long and painful exercise.

If you can make a backup of the server and restore it on another machine (network-isolated from the internet and the actual server, so as to not interfere) then you can pound away on the clone to your hearts content without risking downtime to the production system. That would be a good first step.

After that, though, you're looking at reading an awful lot of "Dummies" books.