Rey's rough upbringing made her strong as a character. She's no expert, but she knows a lot about technology and how to fight due to her roots as a scavenger. She's clearly strong with the force and is able to listen to it in times of need, but there's still a lot of questions around Rey's past that we don't know about. I'm guessing this has something to do with people fleeing the empire back when they were at their strongest.
As for her fight with Kylo, he isn't a sith lord. He's an apprentice who was badly injured before they fought. Commander Snoke seems to be acting like a sith lord, but again, we don't know the full story behind him.