You have to keep on practising, especially for add math & math subject. When teacher is teaching, pay good attention and don't dreaming. Finish all the homework. For extra self practice, buy a good workbook to practice further, anything don't understand can go ask teacher.

For the science subjects, I think physics is most difficult of all, 2nd is chemistry, easiest is bio. Memorising work won't help much here, understanding is the key to score! Understand the concept, get the idea, if just merely memorising its like half past six, very dangerous.

if want to go tuition, normally people will go for add math & physics, which are more challenging.

Just remember to pay attention in class, take good notes, do homework, put extra effort do extra practice, and not shy to ask teacher.