Easier said than done, initially look into the documentation for your router into setting up port forwarding to port 80 and the IP adddress of your ethernet shield on your lan. Once you've got that far you need to know how to address it from the internet. If you're lucky enough to have a static IP address from your ISP (unlikely), its just a matter of pointing a browser at that IP address. Otherwise its going the dynamic DNS route with sites like http://www.dyndns.com/ and something on the inside updating the outside so to speak. Your router is probably the best bet for this. A complex subject in its own right.
The card slot on the ethernet shield is all but useless.
Heres one I did earlier : http://majestic81.plus.com/ 