If you have a broad interest (that is crossing over many science disciplines), I suggest not picking a solid field (chemistry, biology, physics).
Ask yourself which you find more interesting: living systems, planetary systems, or how living systems interact with planetary systems?
If you are interested in living systems go for biochemistry, or molecular biology.
If you are interested in planetary systems go for geology, astronomy, or subsets of oceanography/meteorology.
If you are interested in how the two interact take ecology or environmental chemistry.
Personally I went into chemistry (concentrating on physical and analytical). I ended up drifting mentally because I had so many side interests. Not that what I didn't like what I was doing, just that I could still be distracted by more interesting topics. Namely I was interested by topics that APPLIED all the theories I was learning.
I then switched into geology (technically Earth Science which covered geology/geophysics/hydrology/mineralogy/geochemistry/oceanography/meteorology). This was much more satisfying to me because of the broad use and application of all the other specific fields to understand my main subject.
Studying rocks means understanding chemistry and physics and if one is a Paleontologist then biology too. It is not just looking at a rock. Of course you get the outside adventure of going to get them, which is nothing like lab work.
Lately, as I have gotten into evolution debates, I find myself interested in biochem and molecular biology.
Only if you are truly interested in the general theories behind chem, bio, or physics, regardless of their application, should you go into them.
That's my two cents (plus $40+K in college expenses).
holmes