There are many ways to learn to code these days. From coding bootcamps like Flatiron School to online courses to totally free beginner resources. The best way to learn to code depends on many factors, but the most important one is you. See what course is best for you.
It has also nothing to do with C (and other) language, you need a library which gives you the appropriate functions and data structures.
If you find such a library which supports Windoze and AIX, you code might be straight forward and without platform specific defines.