i want to learn javascript for my OS (made in snap) but i dont know where to start
All the tools you need to make an "OS" in Snap! are already there. You don't need to know JavaScript to make such a project.
You can learn JavaScript at khanacademy.org. Not all aspects of it works in Snap's JS block, however.
You should not learn javascript to mess with snap, instead you should learn javascript for building a website, which there are many many resources available online for that. (there's nowhere to learn javascript to mess with snap, other than reading the source code)
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:
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.