What is the academic route to study Archaeology? I am really fascinated by ancient civilizations and cultures.

I do not know what subjects one has to take up to study Archaeology (like what subjects in A Levels or High School) at an academic level?

I would just like to know in general but would appreciate any details about United States and India.
It is very difficult to answer with very little information. Archaeology is a very vast transversal field. It can be decomposed by techniques, by periods or by the targeted cultures. So the first question you need to ask yourself is whether you want to be an archaeologist (expertise in techniques) or a specialist of a period or culture (egyptologist for instance).

In the first case, scientific majors in high school is probably better than others, but this is not very important after all. However, be advise that excavation techniques are related to "dirt on the hands" techniques, but also to some top notch scientific techniques (specially for the analysis, where you may need knowledge of stats, paleontology, zoology, botany, chemistry, etc.).

In the second case, I would advise to start quickly to acquire a strong background in art history and ancient languages (starting with greek and latin). I will be easier to learn linguistic and semiology afterwards. I guess that both art history and ancient languages can be learned at the high school level in most countries.

At the university, students usually progress concurrently in archaeology techniques and culture specialization.