What we see/know:
-Rey (the girl) is a scavenger
-Hooded figure standing over Vader's mask on a pedestal
-Alien dude passing Luke's old lightsaber to what looks like a girl
-Rey (and Finn) being chased by TIE Fighters
Theory:
-Rey comes into possession of Luke's old saber
-The hooded figure with Vader's mask is the Sith dude - (Suppose it could be Luke but I'm not sure Luke would take Vader's head home after the funeral - I actually hope it's Luke just so my theory is likely wrong, not because it couldn't work but because not having a clue is better)
-The Empire/Sith are chasing Rey in order to get Luke's saber for some nefarious reason (probably to create some kind of crazy Force weapon using a bunch of Force significant objects, including Vader's mask)
As an aside, some are presuming that Rey is a Skywalker or a Solo due to the lack of a surname but, being familiar with how JJ operates, I reckon that's a red herring. Finn might be a Calrissian but I reckon she's just some girl and any Skywalker or Solo Jedi kids haven't been shown yet.