Linux
This chapter is mostly informational. It is not necessary to read it but it is helpful to know about Linux as you will have to use it during your studies.
Linux is a free and open-source operating system. This means that anyone can view the code that drives a Linux system, unlike Windows or macOS which are closed-source.
Linux comes in many variants called distributions or distros for short. They vary only slightly, it is mostly a spectrum of ease-of-use vs freedom of configuration. Some of the easiest distros to install and use include Ubuntu and Fedora, while some of the more customizable include Arch Linux, Gentoo, and nixos.
If you are interested in getting started you can either choose a more user-friendly distro such as Ubuntu or jump in knee-deep and start with something like Arch. People will tell you that one is superior to another, but the true charm of Linux lies in that it can be whatever you want. It is easy to change distro and most people try at least two or three before finding one that fits them the best.
Many computer science students use Linux as their day-to-day operating system. This is extremely educational but it is not necessary. It is possible to have both Linux and Windows/MacOS on one system and there are also other options which you will discover in the next chapter.
If you are eager to get started look at the website of one of the linked distros or try grabbing an older student and asking them for advice.
Differences between operating systems
There are three major differences between OSs.
First, and most obviously, they have different desktop interfaces and therefore also look different, although it is usually possible to make different systems very similar.
Secondly, the programs they can run are specific to each OS. A program for Windows can't be run on a Linux machine, and vice-versa. There are however workarounds to this.
Hardware support is also different between operating systems as they require different drivers.