Luna Paint features a walkthrough experience to learn how to use the extension. Access it via VS Code's Get Started page or the Luna > Help: Open Documentation command.
There are a growing range of tools available in Luna paint which can be accessed in the Tools window in the top left of the image editor.
Image and layer commands
Luna Paint contributes many commands that are accessible via the command palette or the Menu button in the top left corner of the editor. Try searching the command palette for luna image or luna layer.
This is also a great way to learn the default keybindings.