Create a new settings/siteaccess (copy of an existing one) Modify the site.ini.append to your taste (mostly the design)
Based on your setup (virtual/non virtual...) you also have to modify /settings/override/site.ini.append To make the link between your new siteaccess and the domainname/url you want for the new site.
Thanks, it works! But...is it possible to log in to the administration and from there choose which site to work on, or do I have to log into each site separatly using different urls?
That depends on if they share the same db. If they do you can have one admin siteaccess for both, and this also makes it alot easier to share content between sites, but needs a bit more setup.