What you need is called Network Address Translation (NAT). Check if your router supports this, if it does, life is good. Most of them do, so you shouldn't have a problem. What you'll then need to do is read your router manual (or tech support website), which will tell you where to go to do it, then just assign all incoming requests to port 80 to the ip address of your server.
eg,
Router External IP = 61.9.128.16
Router Internal IP = 192.168.0.254/24
Server IP = 192.168.0.1/24
NAT = 61.9.128.16:80 -> 192.168.0.1/24
it's that easy! 🙂