Start by adding important features like apps or programs, or maybe games. There are virtually infinite possibillities you can implement when building an OS.
You might want to get inspired by exploring the features of your computer, and adding features you think you can add.
If you need help, just ask for help and people should show how to do it.
A bit of suggestions:
Add a wallpaper by adding a background to the stage or drawing it.
Use scenes for individual apps or programs (if windows are too difficult). Create a new scene by clicking
New scene in the file menu and switch scenes with the [scratchblocks]switch to scene [ v]::control cap[/scratchblocks]. You should see an arrow on this block and click to send a broadcast. The other scene should receive a broadcast. Don't forget to add a close button that switches the scene back on the other scene otherwise users can't go back.
If menus are too difficult, open the file menu and click
Libraries. Then, select Menus and import it. You will figure out how to add menus.