When given the opportunity to save Mia or Zoe, simply choose Mia by pushing (X) facing her and it will change the ending for you. This is popularly considered the "better" ending of the two.
*** Spoiler - click to reveal ***This is the happier ending of the two because Mia at least survives in the end. Zoe's fate is left to the Baker's residence but we can assume she dies because she appears in a green-like vision where Jack is as well (who you just killed). Also there is DLC called "End of Zoe", it seems pretty self explanatory.