Your sprite experience must be full as well before you can evolve your guardian sprite. You can check your sprite's experience but clicking on your guardian sprite(F6), Clicking the "Details" tab and holding your mouse over the blue/dark brown bar(depending on how much sprite experience you have).
Once the experience bar is full and you have reached level 21, there will be an auto quest which Baden8572 has stated, which will require you to talk to Yobelia. Yobelia is located in Ilya(X 356 Y 70).
iChipster - 80 Demonologist Chippies - 97 Druid