Once you've unlocked the ability to shout from killing your first dragon and retrieved the horn of Jurgen Wind-Caller, go back to High Hrothgar, speak to Arngeir (if he's not inside, he'll be at the tower overseeing the mountain, outside the building), ask him if he's found any words. He will give you a radiant location that hides a word or sometimes a full shout. Go there, clear the dungeon, learn the shout, and repeat until you have twenty. Little tedious but not too bad.