Portforward.
Obvious?
Not always needed...
You can use UPnP to host servers now instead of port forwarding. To check if UPnP is enabled go into your router settings:
Open any browser...
In the address bar: 192.168.1.1
By default: Username: admin - Password: password
Then there should be a tab called UPnP. Now enable it, and just check if anything would ever stop it from working.
Now also make sure the in-game port to 28000. (By default, it should be this.)