You need to buy a server from a hosting website. They end up costing a lot of money. If you have a credit card and stuff, I suggest you run it off a Raspberry Pi. A Raspberry Pi is a micro-computer that allows you to do come cool things (Make emulators and some other cool stuff, search up Raspberry Pi projects to check some out) I'll give you some links to the Raspberry Pi and how to setup a Minecraft server if you are interested!.
Raspberry Pi:
http://www.element14.com/community/comm ... bundle-ban (scroll down and go to the one that has a picture of an SD card on it, it has all the stuff you'll need)
How to Setup a Minecraft Server on the Raspberry Pi:
http://www.stuffaboutcode.com/2013/09/r ... erver.htmlHope that helped!