Okay, Jesus.
SPOILERS AHEAD PEOPLE, DON'T READ BELOW. SERIOUSLY.
We know nothing about Rey's past. Basically nothing. We don't know who she really is, who she was before she was dropped off on Jakku when she was a kid, who her father/mother is/are. Nothing. All we know is she's a junker, and a pretty good one, and has presumably been scavenging parts from shipwrecks for a while (at least 10 or so years, I don't recall them ever saying Rey's age - let's assume at least 18 and she was dropped off on Jakku at maybe 8 years old).
Ok, so when can ASSUME, that scavenging shipwrecks for parts over the course of years would also imply that she knows some shit about ships. To understand a component of a ships worth, you have to probably understand what it actually does and means to the ships functionality. Still with me? So I don't think its so far fetched that she understands how the Millennium Falcon works, or how to fix it or pilot it.
And the movie is called "THE FORCE AWAKENS" for fucks sake. That can mean a couple things. The Force was lying dormant inside her, waiting to awaken. It could mean the force chose her to combat the dark side since Luke was in hiding (and the other jedi had all been killed). It could mean that someone, presumably Luke, hid her on Jakku, suppressed her powers, and in her great time of need and in the face of someone like Kylo Ren, it awakens inside her (hence the kind of random ability to jedi mind trick the guard and deflect Kylo's attempts at extracting information).
If I had to guess, she's a Jedi descendant. Maybe Luke's daughter, maybe Obi Wan's, who knows. She could even be Han/Leia's daughter for all we know and they managed to hide her from Kylo or some shit, and for some reason he doesn't recognize her (just like the Luke Leia shit from the originals, kinda sorta). But whatever it is, I think she had been trained under Luke at one point, he hid her on Jakku and kind of MIB'd her mind of all previous training, and when she finds his Lightsaber the "force awakens", and she starts to remember some of her training, even from such a long time ago. And the reason she is able to beat Kylo Ren is because the Force itself is lending her additional strength, combined with her rage since she thinks Finn's been killed (and she loves him, duh), AND Kylo's injury and confidence issues via Vader's shadow - allows her to take him down.
So TL;DR - there's probably a lot more going on with Rey's character then her just being Goku or some shit. It's such a stupid thing to just outright call her a Mary Sue. Yea, she's totally badass and adorable as all hell (Rey and Daisy Ridley :D); but Mary Sue? Time will tell.