[simonblack]

For the 'feet wet' question:

Some people have suggested assembly. That would be a good idea if we were still back in the simpler, less complex, 8-bit era of the Z80 or 6502 CPUs.

In today's world, I'd go for the "high-level assembly language" or in proper terminology, the C programming language.

To learn that, I'd buy the book of the same name: "The C Programming Language" by Kernighan and Ritchie.

Then follow the procedure laid out in my other comment.