Year 7 to 9 don't matter. Your set in year 7 will not impact your GCSEs, let alone your A-levels.
Different schools have different systems. Ask your teacher rather than us.
Typically if you just display that you are "better" than whatever set you are in, you'll able to be moved up (either by asking yourself or by them asking you).
Dude, you're in year 7, stop worrying. Sets really only matter in year 9, you've got all the time in the world to move up. I was in set 2 in year 7 and moved up to set 1 in year 8, ended up getting an 8 in maths. My main advice for you is to just enjoy being young and not having to worry about stuff like this just try your best