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.
You can bounce traffic around, but it might end up making more sense to just have the service hosted.
If you don't even know why you need to open ports you probably also don't know the risk you're taking. normally zero ports are open. malware crawls the web constantly knocking all ports. if your security is weak then you will get compromised quickly.
Even if you get a private IP on the WAN side of your router (what I can't image, but anyways) your traffic will be translated at some point in the ISP network, so your public IP is exactly the one you get via whatsmyip. This
IP might change over time - typically every 24 hours.
How to open ports on your router depends on the model, so you need to read the manual for it.