As a 6 year veteran of the games industry, my best advice for someone who wants to be a game designer is simple: make games. Make games every day. Make board games, make flash games, invent physical games played with sports equipment, whatever. The medium is less important than just spending time designing fun games and then watching real people play them in front of you. Make a mod for an existing game or engine, make your own twist on a classic game like Tetris or even chess. Just make games.

If making games isn't something you'd do every day for fun anyway, then it's not the career for you.