Firstly, you need to have the Server on a machine that Has a permanent IP.... that means you dedicate an IP in the Routers Subnet, say like 192.168.1.20. Assign that IP to your rig.
Open the Routers configuration page by typing in 192.168.1.1 in your browser.
It would have been a great help to know which router you're using but in anycase most of them have the same layout.
Then you need to find a page in the setting which says virtual servers or ports
There you need to dedicate the PORT and IP to your game name or anything so you remember that's the port for the game.
If you have a Firewall, then you need to dedicate that port and assign it as open in all the firewalls. ie , windows, router, and ISP router.....