The official site https://www.raspberrypi.org/downloads/ contains several distributions for downloading. The two basic operating systems that we will analyze later are Raspbian and NOOBS. You can see what the desktop environment looks like in the next image. Both Raspbian and NOOBS allow you to choose from two versions. There is a full version of the operating system and a lite one. Obviously, the lite version does not contain everything that you might use, so if you intend to use your Raspberry with a desktop environment, choose and download the full version.

On the other hand, if you intend to just SSH and do some basic stuff, pick the lite one. It's really up to you, and of course, you can easily download anything you like and rewrite your microSD card.