It's not easy to give you a step by step guide without knowing what Server OS you are using, what Web Server software you are using, or what Router/Firewall you are using, but basically what you want to do is go onto your router and forward port 80 which is the port used for HTTP traffic to the IP of the machine that holds your website. You tell whoever is hosting your domain name to point it at your external IP (which is technically the IP of your router). If you have any firewalls in place port 80 needs to be open on them aswell. Post back if you needed further help.

It's not easy to give you a step by step guide without knowing what Server OS you are using, what Web Server software you are using, or what Router/Firewall you are using, but basically what you want to do is go onto your router and forward port 80 which is the port used for HTTP traffic to the IP of the machine that holds your website. You tell whoever is hosting your domain name to point it at your external IP (which is technically the IP of your router). If you have any firewalls in place port 80 needs to be open on them aswell. Post back if you needed further help.