how to get A's in my National 5 subjects

Start your revision early on, maybe between now (October holidays) and Christmas depending on when your prelims are.
Past paper questions are a must and you should study as many papers as possible for each of your subjects and are available free on the SQA website alongside Specimen papers for each subject which should also be looked at. You are lucky in that there are now 3 past papers for you to work with (without thinking of the Standard Grade Credit/Intermediate 2 papers you could look at, although some of these courses have some fundamental differences to N5), whereas I only had the 1 past paper. You could also look around the internet and see if any other sources have made practice papers, and Hodder Gibson make books of these if your happy to buy them (they were pretty good at Higher, can't remember about N5).
If your subject has coursework then make sure you spend plenty of time on these as they can make the difference between a lower grade and a better one. English, especially, is a good example of this as you often start your folio near the beginning of the school year so you can keep going back to it every so often and refining it until the deadline.

If you want to concentrate your studies, then focus on English, Maths (getting a A-C grade in these gets you into most uni courses) and then any other subject which either relates to what you want to do at uni/career or the subjects you are continuing into Higher.
Remember as well to take regular breaks, etc. You still have Higher to do afterwards remember.
Good luck!