Linux

My answer to such questions will always be Debian Stable with the Xfce desktop. https://wiki.debian.org/Xfce

Debian is the basis on which Ubuntu (and in turn everything based on Ubuntu) is built. If you take Ubuntu, remove most of the bullshit, add a lot of well-written help documents, and do a lot of QA work and bug-fixing, you end up with Debian Stable. The only real downside is that with Debian you really need to read the installation guide. Where other distribution's installers just do things to your computer without your consent, the Debian installer will ask a lot of questions, and the installation guide gives you the necessary knowledge so you can give an informed answer. Another issue with Debian is that some default security settings are rather restrictive, and you might need the administrator ("root") password more often than you'd like (and be it only to change those settings...).

That said, there are more lightweight distributions than Debian, and more lightweight desktops than Xfce, but I think Debian with Xfce is a good compromise for a beginner, when it comes to the balance between being lightweight and still user-friendly. (I personally use Gentoo on my desktop, with the Sway window manager. That's way more lightweight, but Gentoo is a lot more complicated to set up and maintain than Debian, and Sway is, well, let's just say you need to write a configuration file in order to use it.)