1) Before attempting anything else, try pigging your server from another computer within you LAN and see if you can PING it successfully.
2) If you are able to ping your server internally (within your LAN), then try browsing your website from another computer on your LAN, if you can see the website than we know that internally everything is working fine. If you can't see I would check the firewall settings on the server making sure it's allowing outbound connections on port 80 and then try again. I would also assign a static IP address to your server rather than obtaining it dynamically from your DHCP server.
Also its very important to make sure your webserver is running on the correct TCP Port, by default it should be on port 80.
3) After ensuring that you can browse your website internally, on your router make sure you have a port forwarding rule set in place. If you don't have one, users from outside your LAN will not be able to reach your webserver at all.
4) Once the forwarding rule has been created, trying browsing the website from outside your LAN and see if that works.