Think about how your character would feel as the new kid at school. Decide if your character is outgoing or nervous or some combination of both. Decide how much your character wants to fit in and how much she wants to be herself. Make these decisions by reading the script and analyzing why Gabriella behaves the way she does. With these things in mind, decide what your character wants from the other characters in the play.
Write a character bio. Think about Gabriella's life before moving into town. Imagine her childhood memories and the things that are important to her. Keep a journal to record your thoughts. Be sure to keep this journal separate from any personal diary you may have. Include what you think she looks like, how she walks, her favorite color, etc. The better grasp you have on Gabriella's life outside of the play, the more reality you will bring to her onstage.
Practice the songs and choreography. Learning the moves, notes and words correctly is one thing you can do right or wrong. Respect the choreographer by learning the dances as he taught them; respect the author by learning your lines verbatim. Once you've got the basic outline down, try different tactics to achieve your goal in the play.