I assume you are working with ENet.
In that case if your setup makes one of the players act as the server, then one solution is to have this player(server):
1. forward the used port on their router
2. Make their local IP static,
3. share his public IP address with the others so they can connect to the game.
You can look into port forwarding and have an instructions scene where you explain it to users.
Another case is where you made your server independent from the clients. In this case you need to:
1. make sure the scene structure on your server matches that of the clients in order for it to work correctly
2. host the server on your PC and use your public IP every time the players are trying to play or use a DNS service (for example noIP) so that the url of the server is the same and so the players wouldn't have to get your public IP every time,
3. or you can opt for a hosting service of your choice.
I hope it makes sense, I tried to sum it up, but this should set you off on the right track.