There's 3 episodes for each world in the game:
Toy Story
The Incredibles
Cars
Ratatoullie
Up
Finding Dory
Just complete any episode/stage from any of these worlds and you'll get the achievement... if the game wants it to pop, the game is glitchy with achievements so you may not get this right off the bat, due to the game limiting you to only one achievement after beating it. Normally, you could play as Buzz in Daycare Dash to force the achievements to pop, but for Fast Learner, that won't be necessary, just enjoy the game until all the easy achievements like this one pops.
I also recommend not playing these stages until you get at least most easy achievements:
Daycare Dash (Toy Story)
Fancy Drivin' (Cars)
Canyon Expedition (Up)
Save Metroville (The Incredibles)
The reason for this is because these stages have achievements that only unlock during the end of a stage, which may make things a little more difficult for getting the remaining easy achievements that you should have unlocked by this point. So for now, do everything else in the game and you should be good.