open the syllabus
i would prefer you to begin with optional subjects as to not waste time later on deciding which subjects to opt for
decide now that what optionals are you going to choose after reviewing the syllabus.
if you dont want to go through all of syllabus just check the syllabus of the subjects that you have interest in then choose your subjects.

after you have decided pick one optional and search its topics over the internet get the basic idea of it write it in your own words or just paste it in microsoft word just to get going then when you have gathered ideas of the topics and when you will read the books you will be able to grasp the concept more easily. highlight the additional information on the book and when you have enough information on one topic shorten it in your register.

there is also another strategy to get started is to open the past papers and solve those questions by researching on the material asked in the question and write it down in your register 6 to 7 pages of an answer are sufficient.
this will give you a sense of accomplishment when you have completed the questions. even doing 2 questions a day from past papers are a lot of practice
when you are done with the question have it checked by a credible teacher to get feedback thats how you will improve. best of luck