How can I learn programming by porting a Wii homebrew game to PC and Mac?

A good approach is to replace one system at a time instead of trying to make the entire project work immediately. Start by identifying the Wii-specific rendering code and replace it with a very basic display on your target platform. Then handle input, audio, timing, file access, and other systems one by one. Temporarily disable features you don’t need so you can get a minimal game loop running and see progress.

Be careful not to equate simple gameplay with simple code. A port may require months of changes before the program even produces visible output, which makes it frustrating as a first project. You can still use the game as a long-term goal, but start with smaller programs and approach the port as a series of isolated experiments.

Before attempting the port, build up your programming fundamentals and make a few small projects using the target platform’s APIs. You’ll need to understand the original system’s APIs and then find equivalent functionality on Mac and PC, often writing new glue code to connect everything. A simple Hello World program, a basic window, input handling, and a small graphics demo would be better beginner exercises first.

This is probably much more advanced than it looks. Even with source code available, you may need to replace Wii-specific APIs, rendering, input, audio, file handling, build configuration, and platform assumptions. The gameplay may be simple, but the underlying porting work can be extensive.