SPOILERS FROM STAR WARS: THE FORCE AWAKENS BELOW!
A source has let us know about Leia’s first appearance in Star Wars: The Force Awakens and how the Resistance knows what happened to Poe after the Jakku village massacre:
Leia’s first appearance in Star Wars: The Force Awakens occurs when Brance steps in and lets her know that she has an urgent message from Poe Dameron who needs to be rescued after the Special Ops TIE Fighter he had stolen crashed, marooning him on Jakku. This is also how Leia learns to send an emissary to Takodana and intervene in the story’s events.
That solves the mystery of what happens to Poe Dameron after Jakku (to an extent). It is an assumption by me but it makes sense that they send a rescue party to pick up Poe who then readies Black Squadron to stop the First Order on Takodana! This is also Threepio’s first appearance too, from what we have heard.