I second the reading part. The best way to hear how teen characters sound is to read teen novels. With fantasy, there isn't that much difference between YA and adult, so you can probably rest easy there on how the characters sound. The odds of your MC sounding too old are pretty slim, unless you have her knowing things that would require someone with a lot more life experience to understand.
Pay attention to what they know and what someone in that age range would know in your world. If it's fantasy, there's a good chance kids grow up faster and have to take on more adult responsibilites earlier, so sounding older wouldn't be a bad thing. 16 was "adult" in a lot of cultures. Heck, it still in today.
There really are no "do nots" in YA these days. You'd want to focus on problems and stories that would appeal to a teen audience, so you'd probably want to stay away from stuff that would only appeal to adults. Though honestly, in YA fantasy, I can't think of anything that would be adult only except graphic sexual situations and violence. You can still have sex and violence, you just need to back down some on the details. Go more for the emotional aspect than the descriptive.