I have no idea what you want to do. If you want to run eg Minecraft server - install VPN client on the server, connect to the service, and let your friends connect to the IP address assigned to you by the VPN service. Once this works, you move the client to the router (may be get a new router), set the client there, and port-forward the VPN IP address to your server.
I can't be of any more help, and you are up to a lot of reading... And if you can't figure it out, may be the easiest way would be to buy a public / static IP address from your ISP.