If you're reading this, there's good news. You've probably already starting doing all the things you need to do to be a designer. Bag of Doritos in one hand? Highly caffeinated beverage in the other? If you answered yes, you're already halfway there. First and foremost, you need to play games. You need to love it, and we're not just talking "I root for the Bears when they're winning" love... we're talking "I wore my Mike Ditka signed boxer shorts to my wedding" love. If you don't have that, you'll probably get tired of it pretty quick.
Secondly, aside of actually playing the games try creating some. You don't need to know how to program to do this. Write a killer quest out on paper. DM a game of Dungeons & Dragons for a group of friends. Make mods for NeverWinter Nights or Morrowind. Write short stories. All of these things will help you develop the ability to get creative ideas out of your head and translated into compelling gameplay ideas and content. It's also provides a great response to angry parents/spouses demanding to know why you're always on that infernal computer.
http://www.mmorpg.com/gamelist.cfm/game ... ature/1046