If you're setting up a standard website, forward port 80 to the internal IP address of the machine behind the router. This will allow you to access the website by using the public NAT address of the router (you can get that address by visiting www.whatismyip.com
from a machine behind the router).
If you want to access it by URL, assuming you have a single dynamic IP address from your ISP, you'll need to use a service like www.dyndns.com
or something similar. If you have a static public IP address, you can just register a domain name and make sure the public A record points to your public IP address.