What i mean is, when you go to a word wall, let's say you get Fus. you kill dragons to obtain their souls to upgrade a shout of your choice, in this case you would get Ro for the first soul spent on this shout and Dah for another soul. To get an entirely new shout, however, you must find a word wall, which i am sure will NOT be random, similar to daedric shrines
Im pretty sure it works this way:
Find word wall
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Kill dragon
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Find another word wall
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Kill dragon
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Find word wall
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Kill dragon
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(although you may kill a dragon and get its soul first I just used this order because it was easy)
Edit: that puts the word walls at 60 in the game and that figure sounds more correct to me.