Um, you're twelve... but ask her out, I guess.
I'm 12 too, idiot.
Telling her how you feel will result in an unhappy ending, almost always. You should just be good friends with her, nothing more, until someday, if you both have been friends for years, and you know each other like the back of your hand, you should tell her your true feelings.
Wait, do you like her personality, or her body, or both?