Application date and tutorial to do the exam?

Hello, I am an A-level student studying at a private exam center/school, so they don't allow retakes and such for A-level graduates. Hence I'm looking forward to doing more subjects through self-studying A-Levels and doing them abroad.
However, problem is, I barely know how any of those works. As in the real life process. Not the payment and registration at schoolexams.britishcouncil.org. But the entire process, the travel and journey. Even which paper to practice.
I'm currently doing A-levels based on the Pearson Edexcel IAL exam board, but also will do future subjects from the Cambridge board. Anyone willing to help me out here?