For a simple start you can copy ‘n’ paste code into the Ruby Console (Extensions → Developer → Ruby Console). Notepad, Notepad++, or VS Code will be fine for writing code.
Otherwise, if you want to create an extension you generally need a registration file (e.g., “MyExtension.rb”) and a main file (e.g., “Main.rb”). The registration file goes into your Plugins folder (C:\Users\username\AppData\Roaming\SketchUp\SketchUp 2023\SketchUp\Plugins) and the Main.rb goes into a folder that you will create (with the same name as your extension, e.g., “MyExtension”).
Dan has provided examples. Here’s one: [Template] for Basic SketchUp Extension - Developers / Ruby API - SketchUp Community
Or,
If you just want to test and keep scripts, you can also use the Toolbar Editor extension to save and run scripts.