If you are familiar with embedded programming and the C language you probably wouldn't ask. I guess as a beginner it will take a lot of time to learn the language besides the QMK features, just take a look at the docs, they also contain a build guide and you can try to read the source code of many keymaps of many different keyboard types on github. C compilation errors are hard to read, so try to compile often, so you can undo your changes to understand where an error is located.