If you want to prepare for Maths and Chemistry (HL Level) then the most important point is to understand the IB syllabus for both the subjects. You need to be aware of the subject and topics covered in it.  Both maths and chemistry are about understanding and practicing, so practice as much as you can.

You can also solve question bank and previous year papers to check your knowledge. Make a habit of following schedules and be responsive. Brush up your writing skills and enhance your critical thinking by questioning. Enhance your creativity and participate in extra curricular activities and outdoor games.

Balance your time through time management as you will have to struggle with workloads and deadlines. Revise whatever you have learnt after completing a chapter and solve question related to the chapter.

First of all, there is no need to do an advance preparation for IB subjects as the subjects are taught with a holistic approach. But if are really interested to prepare in advance then check out the syllabus of IB maths and chemistry either online or from your school IB counsellor.

Try to understand the topics and polish your writing skills by practicing essays. IB focusses a lot on writing skills so a student needs to practice a lot and also because he will have to write lots of lab reports and experimental results. For this, you can start practice by writing small paragraphs first and then moving to extended and longer essays. Participate in extra curricular activities since IB also has CAS (creativity, action, service) as one of its core elements.

Try to inculcate the habit of doing research by studying paper work, research materials from suggested resources. Finally, revise the basic concepts that you have been taught.