Do not change the IP address in the server.properties file. It will not work as you have not 'claimed' the IP for your own use. Look up on Google, 'What is my IP?' and you should use the public IP address. The port forwarding is as simple as punching in 25565.
First I downloaded the 1.12 Minecraft Server Version, and booted up the server, but I did not go on it. I changed a few of the server properties, then asked my friend to try to come on. When I was telling him my public ip I realized it had changed. I typed the new ip in for my Minecraft and it worked.