Hi Erin! Congratulations on being a senior soon -- high school can be tricky both academically and socially, but it's also an experience so enjoy the good parts while they last :)
For me, at the root of it all I knew I excelled at math and science in school and that those were the areas that I enjoyed learning the most; from there I wanted to go into engineering because of the people I knew as I grew up -- my dad is a former chemical engineer, my oldest sister got a degree in Civil, among other influential people. They reinforced the areas of my personality that fit in with engineering -- analytical, problem-solving, hands-on learning, etc. Engineering was just something that fit me well.
Materials engineering was something I discovered as a senior in high school; before then, I was assuming I would do mechanical, which is a broad, general engineering major that can go into a lot of different jobs and fields.
It's hard to know where exactly you want to end up, and people's plans and careers change whether they intended to do so or not. So consider both what your ideal career is (what would get you jumping up every morning and looking forward to the day) as well as what your options would be with a given degree. As I said, mechanical is something I consider to be applicable to a huge variety of fields; material science in its own way is too -- I thought I'd go into biomaterials or nanoparticle research but ended up in a sportswear testing lab!
I would encourage you to talk with people who: a)have experience in the industries you are interested and ask what they majored in; or b) alternatively, ask people who have degrees in the academic departments that you are considering what industries and jobs are open to them. For example, the people in my lab here have a variety of backgrounds: materials, mechanical, chemical, and bio-engineers. Different backgrounds and career paths, but clearly we all ended up in the same place.
With regards to picking a specific engineering over all the other kinds, there is some overlap in required classes for the various engineering degrees. As part of my materials science and engineering degree, I also took classes in chemical, mechanical, civil, and computer science engineering. So if you go to a place with more than one engineering major, you should be able to get a taste for the different ones and maybe one will inspire you the most. This is also true about college in general: you don't have to have absolutely everything figured out and nailed down before you ever set foot on college campus. Your experience while there will influence and shape the choices you make, as it should.
Best of luck! The fact that you're thinking about your future and being conscientious of where you want to go in life, I know you'll do great at whatever you set your mind to.