I say talk to your school councilor or whatever you guys call them in the US. They will help you and guide you into making the right course choices for the right program you want to take in university. From my experience, computer (sciences) is more college orientated and easier to get a degree in. On the other hand you will have better job opportunities with an engineering degree, but it will be harder to achieve and it is obviously a university thing. As for sciences, it's a really big field going from biomedical sciences to aerospace sciences, so you can't say you want to study sciences because it is simply to vague and a big ass field. As I said before, I would highly recommend to talk to the school councilor because helping you choose the right courses for the program you want is their job. Either way, you would have to take chemistry, biology, physics and multiple mathematics courses in High School in order to apply for any of the 3 fields you stated above. I don't know how universities look at applications in the US but in Canada, they only look at your top 6 grade 12 class grades if they are eligible. And when I say eligible it means that they won't look at your grade 12 Spanish(example) grade if your apply for a mathematics orientated program. As for college they simply want you to have your High School diploma and possibly a certain grade avg. Feel free to add me to ask more questions.