How do I use shout power levels?

Solution 1:

You have to hold the Z (PC), R2 (PS3), or RB (Xbox 360) key for a while to charge the shout and use more words.

Solution 2:

The longer you hold the "Shout-Key", the more words of the Shout will be used. As seen in this wiki, the cooldown is differs. For example the first shout you learn. If you use it just for a moment, you only shout the first word. Cooldown for this is 15 seconds. If you hold it longer you use two words (cooldown 20 seconds). Holding the button even a few moments more, will let you shout all the three words, wich will also set the cooldown to 45 seconds.