The first time 你 ever see her is early in season one. Before they ever leave for Duelist Kingdom, she sends Joey a video tape message. The first time 你 see her in person I think is early season two. And I'm not sure, but I think she's about 14. (I know this isn't a great answer, but hopefully it'll hold ya over until someone who knows 更多 can post an answer. XD)
Serenity first appears in episode 2 of the first season, "The Gauntlet Is Thrown." She sends him a videotape letting him know how much she misses him, and she tells him that she feels like she doesn't have much time left, meaning that she is losing her eyesight. In this episode, she is 13, and Joey is 16.