The best place for javascript documentation is Mozilla Developer Network: https://developer.mozilla.org/en-US/docs/Web/javascript
I don't know what the best javascript tutorial is, but MDN also has a tutorial in the link above. You might just want to spend a few minutes on different tutorials and find one you like.
For userscripts specifically, you will want to look at other people's scripts as an example, and find the greasemonkey API here: https://wiki.greasespot.net/Greasemonkey_Manual:API Be aware that there are differences between different userscript managers, and greasemonkey versions even (e.g. https://www.greasespot.net/2017/09/greasemonkey-4-for-script-authors.html ).
Newbie needs guidance
windows.alert('Hello World'); I am an aspiring programmer and I signed up on this website to learn how to make scripts form the maverick programmers who are making userscripts here. So please guide/teach me how to get started and what tools and resources i need to make userscripts. Thank you!
PS: I have basic HTML and CSS knowledge and I am trying to learn Javascript.