You can google out the answers to all your questions, but I'll
address the last one for you :
10th percentile and 90th percentile refers to the grades
obtained by the bottom 10% and top 10% of A level students who
successfully applied for the Uni course last year,
respectively.
If you decide to come for my BedokFunland JC
private tuition for A level H2 Chemistry (which may be a
prerequisite for your NTU course, depending on the exact College
you matriculate under), I'll address any remaining queries you have
regarding the A level process in-person.
Even if you choose to matriculate under a College in which A
level Chemistry is optional, but you'll suffer a severe
disadvantage if you do not have A level Chemistry, when taking
Biochemistry modules, so A level Chemistry is strongly recommended
for anyone taking any Science courses in Uni. A level Chemistry is
compulsory for applying into Medicine.
If you're gonna self-study without going for private tuition,
you can post H2 Chemistry questions here if you need help.