One of the hardest things to do with first-person narration (apart from the problem of how to tell the reader about important things that happen where the POV character can’t see them) is describing the narrator’s appearance. With third person, it’s relatively easy — you can slip in a detail here and a detail there as the opportunity arises, or you can say the heck with subtlety and provide a couple of descriptive sentences about the character shortly after he or she is introduced. But with first person, you’re not just following the character around and eavesdropping on their thoughts when it’s convenient. You’re inside their head all the time . . . and most people, unless they’re either really vain or really insecure, don’t spend that much time thinking about the fact that they have, say, brown hair and hazel eyes and a nose that’s just slightly crooked because they broke it falling off a seesaw back in second grade.
So what can you do?
Well, you can always not bother with physical description of your first person narrator. It’s surprising, really, how irrelevant brown hair and hazel eyes are to a lot of story lines. (For the story lines where they are relevant, the narrator will tell you about them — in fact, if he or she thinks that the crooked nose and the unexceptional hair and eye color are why they are still tragically without a date for the senior prom, they are probably not going to shut up about it.)
But what you don’t do (and don’t do it with third person narrators, either) is have your first-person narrator look at themselves in a mirror and describe what they’re seeing. And for “mirror” read also lake, pond, mud puddle, silver bowl, shop window, the eyes of the beloved, or any other reflective surface. Because that has been done.