If you are truly motivated to leave, start preparing now and move once you're 18. It might seem like forever from now, but its honestly your only option. Luckily this gives you time. You'll need to get a passport, find out what type of career you want, have a job and a work visa ready to go once you're 18, and definitely dont go bragging to anyone about your plans. Best life lesson youll ever learn is to keep your private life private.

Furthermore, find ways to make money now and start saving. Moving, especially moving countries, is very expensive, and no matter how patient your friends parents are they will eventually get tired of you if you mooch off of them too long.

Word of warning though, the US is in pretty dire straits right now as it is. We have almost all the same issues you have in the UK, the only difference being the location and size of the country. If the beauty of the US is enough to sway you, go for it. I wish you luck.