Password protected site/admin page to update MySQL db

Hello,

I'm wondering if you'd be able to help me, I'm just looking for a noob-and-efficient way to do the following:

1. Make my site password protected - username should be the same for each user. A guest password that can be changed. Other users can sign up to receive their own password by email, but only if they sign up with an email address that matches a specific domain (i.e. @mycompany.com). Anti-robot checker built into this sign up process.

2. Whenever a user designated as an 'admin' signs in, they have permission to access a page where they can edit a MySQL database, whereas non-admin users can't get to the page.

3. How the heck do I make a page that can allow admins to edit a database!

My web host has a one-click installer for password protecting my site, but I'm trying not to cobble together several things and then after hours of work, find out they're all incompatible with each other and then be back at square one.

I recently tried uploading a search engine script and my host wouldn't accept the .htaccess and .php files so if I need to approach this from the ground up I don't mind reading guides, just need to know where to start.

thanks guys :)

Nothing about that is for someone new to programming! Start here.