You want to give back to the community that has treated you so well and run your own game server. For the most part, though, the points that I'll touch on can be applied to almost any other game out there. The first things that need to be addressed is hardware, and, just as importantly, your connection to the internet. The hardware needed to run a game server depends mainly on three things. First, you need to decide what game you want to run.
Some games are a more CPU intensive than others and require more horsepower.
Secondly, you have to consider what operating system you will run on your server. Granted, this is often determined by the game you wish to serve, but many game developers produce more than one server version of their software. Generally, you can get away with less hardware requirements by running your server on a unix-flavoured operating system. Finally, you also have to consider what your maximum player count will be. Exceed your hardware's resources by setting this value too high and performance will suffer since the server can no longer maintain everything in memory.