The exact steps you need to take depend on the make and model of router you have, but in essense it involves possibly two steps:[list=1]
In your NAT settings (may also be called PAT or NAPT), you need to configure accesses to port 80 (and port 443 if you use https as well as http protocols) to be forwarded to port 80 (and 443) at the IP address of your web server (the IP address by which it's known locally).
You may need to adjust firewall settings in the router to permit traffic to port 80 (and port 443) to be allowed and not dropped.[/list]How you test it once you've set it up is important -- you cannot simply browse to your public IP address from a PC inside your network: this will not work. You need to get someone from outside your home network to try it. Or, use a dial-up connection from another PC (not the one running the webserver) and point its browser at username.force9.co.uk (no leading "www", note -- it is mapped to your public IP address).
If you need specific help for your router, I suggest you post a message on the uk.telecom.broadband newsgroup, specifying the type of router you have.