You don't have to take H2 bio to be a doctor. H2 phy and H2 chem
(the most important subject for medicine) would suffice. So work
hard and build up a good foundation in these subjects, as well as
your Emath/Amath. Of course, English as well as the humanities are
important too, because all-round good grades (and more) are
required for you to get into medicine. If you're interested in
picking up bio in your own time, there are plenty of resources
online, and you could purchase the O-level bio textbook, as well as
'Biology (9th edition)' to read up on topics in the H2 bio syllabus
(and beyond). All the very best!