The dilemma a lot of writers have, including me, is how much character description to put into a story. Too much, and the reader's vision of a character in contradicted at every term. Too little, and you're not giving the reader enough to work with.*
So what to do?
1. I feel like it's important for a reader to have a general idea what a character looks like. I try to sketch a general physical impression of a character--height, weight, body type.
2. I find that if I describe one character whose appearance is important, I can describe other characters in relation to her. Is character B taller or shorter than character A? Wider or slimmer? Darker, or more fair?
3. A description of a character's movements can give a reader an impression of the character just as well as a detailed description of physical traits. Is the character quick or slow? Graceful or awkward? Athletic or something that is the opposite of athletic? :)
4. Physical description should reveal something about the character doing the describing. If a character A is looking at character B, the way she describes him should tell us something about her, how she is feeling, what she thinks of him. It should be more than just physically revealing--it should be revealing emotionally.
5. In a first person story, describing the main character is always a challenge. People don't think of themselves as "fair-haired and delicate, with a snub-nose." And the old trope of the character looking in the mirror (or any other reflective surface) is way played out. So what to do? In YA, clothing and personal style really matter, which means that a character (especially a female character) can describe what she's wearing, and her make-up and hairstyle in detail and be telling the reader something about herself beyond her physical appearance. It's a little more difficult with guys, but not impossible--they care about their own personal style, too, even if they don't have as wide a vocabulary to describe it.
6. Good characters don't always have to be perfect looking, and bad characters don't always have to be ugly. And vice versa--the bad characters don't have to be beautiful but deadly, and good characters don't have to be plain and wholesome. BORING. Characters can be average looking, or just have one good feature, or only be attractive to the ones who love them.**
* And, of course, there are the conventions of genre--romance novels have a lot more character description than thrillers, and literary novels have a lot of description, but not usually of characters (in some literary novels I've read, I literally have had no idea what the characters look like).
** True story: I dated a guy in college who was not conventionally attractive. He wasn't the elephant man or anything, but if you just looked at him and didn't know him, objectively you'd say "that guy's ugly." But if you got to know him, even a little bit, you sort of forgot that he was ugly. It stopped mattering, really, because he was funny and interesting. And if you went out with him enough times to end up kissing him, well, then suddenly he wasn't ugly AT ALL.