Why can't I shout in Skyrim?

I bought Skyrim V, so far its a great game, and I have slayed the First Dragon at Whiterum. But now one of the guards asked me to shout, and when I select the unrelenting force (the only shout I have so far) It says I do not have enough charge for that Item.
So what am I doing wrong? Shouldnt I just be able to do it first try?
Thanks, im on Xbox360 by the way.