I'm making a web site/application and I want to be able to access the website from outside my LAN. I'm serving the site via IIS; When accessing the site from within my LAN everthing looks good, but when I try to reach it through my external IP address (found using http://www.whatismyip.com/) I'm reaching the Router's admin panel.
You'll need to login to your router and forward port 80 to your computer running IIS.
You may want to consider a shared host, there are several disadvantages to hosting at home, which are discussed a bit here:
I digress, this should help you out: