How to sing loud but still go high?

When I sing I try to sing lke Demi Lovato, but I can't hit the high notes right. I'm still only 12 and my voice is lower than my other friends. When I get to high notes, my voice gets quieter instead of staying loud. Why is this happening and is there any way that I can remain loud but still have good tone and hit the high notes. p.s. the only place I can sing loudly without being self-concious is the shower...