How can i leave my grandmas house ?

How can i leave my grandmas house im almost 17 i will be 18 in like 7 months im ready to get out of here, i get called fat all the time by my grandma when im not at all im tall and perfect weight for my height. when my grandma gets mad she takes it out on me and my brother a couple days ago she threw a sippy cup at him and just today she tried hittin me she has 4foster kids also and she always smacks on the babys what am i supposed to do ? how can i get out of here my boyfriends mom is more of a mom to me than my grandma ( my mom ped away when i was 3 ) how can i get out with out no trouble and move in with her