How do you sing like a rockstar if you are a girl?

It's the same for any gender. Just sing as much as you can till you find your own voice, and then start singing the kind of music you want to sing. With rock, it's best to have quite a powerful voice (learning to breathe from the diaphragm can help, and if you've ever done woodwind or brass instruments in the past that will help) and not to be too picky about being perfect.