On top of what Guns and Gins said :

Steam provides the server software free of charge whether you buy the game of not. You can set up your own server , there are many guides available through your favorite search engine or right here on Steam.

You can also just pay a service like gportal $45 for 3 months and you don't have to set-up anything , they give you an ip and port you use to let people join and you can have any rules you want.

I have used both an extra box and the service(s) to host dedicated servers , fairly straight forward.