JK Rowling. A bit cliche for a teen writer, I know, but she really did. I grew up on her books, and I'd always loved to write (although my stories when I was 6 had absolutely no plot, but the characters should had a wealth of conflict), but she was the one that really made me go "I could do this".
Especially when I heard her story of being rejected by publishers and then I saw what she became. And I read her books and they don't seem so high over my level of writing (they are, certainly, but not impossibly so) and they're in a genre I love and it just says to me that I can do it, too.
I dream big. I never let myself think I'd be a mediocre writer. When I start writing, I picture the interviews of actors who play my characters when it's made into a movie. I imagine the interviews I'll give, what people will say when they analyze my books after my death. And JK Rowling, to me, was the first example I ever related with where my dreams were actually a reality for her. She got the movies, she got the millions, everything I dreamed of getting, she had, so it made it possible for me.