How low-level are you talking? You could either start with an assembly language (which is the closest to the hardware you'll get), or something higher level like C.
There are exhaustive C resources everywhere.
For getting started with assembly I recommend following this interactive tutorial for the MOS 6502 (a relatively simple CPU from 1975, famous for being used in the NES, Apple II, among others).
https://skilldrick.github.io/easy6502/