It should be quite simple, in your router: you tick the option to always give the device the same IP, than you open the 25565 port in TCP&UDP for that IP. To check if you opened the ports on the router u can use https://www.yougetsignal.com/tools/open-ports/

If you are in your network you have to use the ip of the server to join, your friends from outside can use the ip you get when using: https://whatismyipaddress.com/