This achievement is story-related and cannot be missed.
*** Spoiler - click to reveal ***This achievement unlocks shortly after "On the Path". Clementine's new rescuers will walk her back to their shelter and discover she has been bitten. After Clementine persuades the older gentleman that she is telling the truth and that the bite came from a dog, not a walker, the younger gentleman will throw a hissy fit (I have a feeling I'm not gonna care much for this guy). As the next scene loads, the "Now What?" achievement will unlock.