For advanced customisation of WordPress, you will need to edit software files.
It is really important to use a simple text editor. A simple text editor does not have any formatting (bold/fonts etc). If you edit website files with an editor that supports formatting, you will likely corrupt them.
Advanced code editors display the various code using different colours, making the code easier to read, understand and edit. They will also have tools to help you write the code.
Two really good free code editors for beginners are: