I've been doing a bit of beta reading the past few weeks, and one of the things I've noticed in all the manuscripts I've read is the lack of contractions. There's nothing like a present day teen saying "I am going out, Dad. Do not wait up for me." to drag me out of the story.
Everyone uses contractions when they speak. If they don't, it's because they are emphasizing something. When you write dialogue, and even in first person narrative, you need to use contractions or it sounds stilted and unnatural. And you don't want your characters sounding like robots, right?
At school you were probably taught that in writing, you don't use contractions. Writing is formal, and you should use the full words. Yeah, that was probably true in the past, but these days, writing is much looser and freer and voice is everything. And the voice feels unnatural if you don't use contractions. Especially in YA.
So don't be afraid to thumb your nose at those teachers you had back in the fifth grade. Writing 'you're' instead of 'you are' is not going to send you down the slippery slide to writers' hell.
Do you use contractions?