First of all, you need to ask yourself if you're going to be able to handle 4 major subjects. A-Levels isn't very easy to begin with and if you take 4 subjects, then you're gonna have to work really hard. I hope you don't think I'm demotivating you, I'm just making sure you're ready to handle four big subjects. I took the four subjects you mentioned and I ended up dropping BS half way through the year.
Nevertheless, the most important point I can give you is something very simple. After every class, revise whatever your teacher taught you. I cannot stress how important that is because chances are, if you don't revise the topic, then you'll end up forgetting it by the time you move on to the next topic. Buy topical past papers, and whenever you finish a topic, try its questions from the past papers. Most kids ignore the past papers until the last few months and end up very frightened because of how difficult they are. This goes for Economics especially. The questions in the Econ past papers are really confusing. These are very general tips but they apply to everything. There's no easy way out of this. Try learning from the best teachers, I personally believe Sir Zarak Mushtaq to be the best teacher for Economics and Sir Ahmed Saya to be the best teacher for Accounts and Maths.
Keep in mind, that because you've shifted from science to commerce, you'll see commerce background students performing better than you. Don't let that demotivate you. I, myself, switched from science to commerce because I lost interest in Physics and I, Alhamdullilah, did well in my A-levels final CIEs.
