I hope you like math!

For starters, you'll need to write the program it runs, in which case you should consult the assembly manual for your target CPU, or, if you wish to construct one custom, you'll need to learn how to design a CPU. Mark2222's excellent subframe tutorials can help with the latter. In both cases, you'll at least want to know subframe basics for assembly.

As for peripherals, i.e. the display, you cannot mix and match peripherals between CPUs, so you will either have to design an adaptor, or design your own peripheral. (see subframe tutorials, and CPU manual for how it's IO works)

First, you need to learn subtraction, multiplication, division, and addition in the binary number system (there are a lot of these number systems, the most popular are hexadecimal, octal, binary and decimal) .I advise you to gradually study this topic, as in the process of studying it may seem that it's impossible. So, when you figured out the binary number system, start studying logical valves and their implementation in a powder toy, then try to repeat the schemes from the Internet (for example, a full adder or a half adder) using those components that you made in a powder toy. I advise you to use Reverse Engineering, it will be easier to learn,