I truely hope you've googled and already have read this: Tutorials/Setting up a Minecraft server
] and especially that interesting part about port forwarding: Port Forwarding
], but if you had there would be no question, would there? For setting up the router you'll have to refer to the manufacturers documentation. I for instance have a DLink DIR-615 which lets you setup everything quite smoothely. You'll have to find out what equipment (routerwise) is sitting in front of your Minecraft server.