As former JC Lecturer I would advise you the following:
1) If is Pharmacy then biology would be helpful, if is Chemical
engineering then probably physics.
2) I would recommend you go with Geography. If you are
interested in Economics you can take an introduction module in
university. Having background knowledge would put you in a safer
position to tackle the subject.
3) If you cannot decide between physics and biology then go
based on your interest. This is not always true but I feel that
having physics can be helpful to A level Mathematics. My students
from biology tend to be not as strong. This is not a fact but just
based on my experience.
In general reference books are not complusory for JC as schools
have their own lecture notes. It is best you ask your JC tutors for
advise. There may be a little "conflict" if the book is not based
on Singapore education and the way of answering is different from
what your school accepts.
To survive well, the key idea is being consistent in making sure
your school work is done and don't fall into the situation too much
time is spent in other areas such as CCAs, etc. You should strike a
balance, and ask questions whenever in doubt instead of
accumulating.
JC life is really more hectic than Secondary school, pace of
learning is also faster.
