Now you are effectively asking for private tuition. That is usually someone's paid job and i doubt anyone here will commit to anything like that.
One doesn't decide to be competent, one graduates into competency. The only way to do so is by climbing the hill from bottom up, as high as you need to go. How exactly you choose to do that is up to you but nobody else can do it for you.
Psi already gave the good advice; don't try to leapfrog into writing a PID control algorithm if you don't have a grasp of the basics. A polished piece of software such as a good PID implementation is a non-trivial thing and once you have written it yourself, there is a lot you understand about real time processing, signal manipulation, numeric computation and just plain writing code. All of that understanding won't just jump into your head, however much you want it to.
If you haven't yet written the canonical blink-a-led program, then that is where you should start. Achieving that momentous result is not as trivial as you might think because to do that, you need to be able to set up the programming toolchain, HW environment, transfer the program successfully to the device and actually run it there. Whatever you can't do in that exercise, there are people here and elsewhere who are glad to give pointers and identify problems. But it is you who need to do the legwork and strain your brain to understand. If that fails then all fails.