First, make sure that you have the httpd deamon running. Second, make sure you can access http://localhost.
If you can, next you need to setup port forwarding to the router. You also want to assign your linux box a static IP within your routers range. This can be done via the network tool under gnome, under system tools I think. You will want to change the IP to 192.168.1.51, subnet mask 255.255.255.0, and gateway 192.168.1.1. After assigning this, make sure you restart your ehternet connection. (If all else fails, reboot)
Go to your routers administration panel (default for linksys: http://192.168.1.1
uname:<nothing> pass:admin) . Go to port forwarding, and forward port 80 to 192.168.1.51. You should know use your IP addy from your ISP to access your webserver. However, your ISP MAY block port 80, so you may have to use another. This would require changing the forwarded port on your router, AND changing your httpd.conf file fo apache. There will be an option to change the port, change it from 80 to another port (I use 82). After you access it from your IP address, next you need to assign your domain name to your IP.
Setting it up with your www.domain.com
FIRST, you need to take your ip address and register it with a DDNS (Dynamic Domain name server) with your www.domain.com
name. I recommend www.zoneedit.com.
Then, with this, you may need to wait a day or two before you can access it. But the DDNS kind of forwards requests to your name to your IP. Then, your router will forward the requests to your linux webserver. Also, each time your get a new IP address from your ISP, you need to update it with zonedit.com.