Starting sentence help PLEASE!?

Okay first things first I'm working on writing a book just because I'm a really good writer and I feel like it, but the beginning sentence always gets me. In the beginning it's about a girl who lives in an orphanage and she is sitting in her room thinking to herself. I was thinking of something like this. Four walls, four huge, unbreakable walls that were meant to keep things in, meant to keep me in. But I don't want her to sound all boring and depressed. I want her to sound independent, funny, and strong, but when she talks to people she is all shy, but in her head she is really very bold. Sooooooooooooo if you could help me go off of that or do something completely different I really don't care any help is welcome. Oh and I also don't know if I should write in third or first person.