Living alone at America?

Ok, so here's a thing. I'm sixteen at the moment, don't think I'm a stupid teenager or something.. I'm very independent (because of my moms illness, taking care of me, and my 6 yr old sister since I was 11) and I'm already thinking about my future.. I'm currently living at UK, but not its citizen (I'm Russian). And I'm thinking to move to America (alone) when I'm legally independent, so I guess when I'm 18.. I'm working part-time job at M&S, and studying Art&Design at college. Can anyone tell me where do I need to start, to have a full life at USA (I'm thinking about living in Seattle). Do I need a Green Card or something? How much money I will need to rent a flat, or a room, and is it hard to find a job? Thank you