Here's some basic info on setting up a server for Co-Op.

To start a server, simply click PLAY COOP from the main menu, put in a username, and hit HOST GAME.

Joining someone else's game? Input their external/public IP into the Server IP field. You can find your external IP by going to Google and typing in "IP"

In order for a server to work properly, you'll need to be sure port 7777 is forwarded. This allows the game to send data back and forth without it getting intercepted by your router's firewall.

Need help with port forwarding? https://portforward.com/router.htm

Need help on setting up a Static IP? https://portforward.com/networking/staticip.htm

I'd also recommend checking out Tunngle if you're having trouble with port-forwarding, as it's a widely used VPN service for allowing people to game together. https://www.tunngle.net/en/

If you're sure your ports are forwarded and you're still having trouble with people connecting, try disabling your antivirus completely to determine if it is overriding your router settings and blocking traffic. ALWAYS BE CAREFUL with disabling AV software, and be sure to re-enable it when you're finished troubleshooting.