In much of mainland Europe, you have to decide what you want to be by the time you start high school, so you know what prerequisites to take to get into the university program you want. In most cases, you get one chance to choose, and then you're locked into that path.
While it is theoretically possible to decide late, it's very hard to do, and the vast majority of people will never get the opportunity to go back and get the prereq's they need, so it's best to get it right the first time. Same goes for changing majors once you're in uni; it's possible, but very difficult and highly unlikely.
I would look up the websites of some American universities you're interested in, and see what their entrance requirements are. It is fairly easy to get financial aid and scholarships/bursuries as an international student, so look into that too. If you want to work in the US, you'll definitely have to go to university in the US. They don't accept internationally trained doctors, last time I checked. Also, apply as early as possible; it can take a while to get the paperwork sorted out, and spaces fill up quickly.