Hi Daniel. It's a very easy process on IIS.
First, create a directory for the new site. This can be anywhere on your harddrive.
Open your Internet Information Manager.
Double click the name of your server.
you should see "default web site" and "administration web site"
Right click on the server name and "add new web site"
The wizard will appear.
give a nickname to your new site, then find the path to the new directory you created.
When it asks for a host header name, you type in the exact domain name you want the site to be: www.mydomain.com leave the IP and port as default.
Now, when someone on the web types in www.mydomain.com it will come to your IP and find that directory instead of the default directory.
There's lots you can do beyond this, but this will allow you to use your one IP address for quite a few different websites.