Now that you have a server you will need to install an operating system.
Perhaps you don't know that the most reliable and popular servers in the world, run on GNU/Linux!
So, you will need to install a GNU/Linux version (called distro, for distribution) on your server. There are many options! And typically there is always a trade-off between user-friendliness and performance.
Here we will go for user-friendliness.
Back in 2021, the Ubuntu 20.04 LTS (Long Term Support) Server edition is perhaps the easiest to install and manage. It is built upon a Debian GNU/Linux distro, adding a number of tools. In short, Debian’s installer gives more options. Ubuntu’s is much more user friendly. Debian has a complex release schedule (stable, testing, unstable), while Ubuntu releases an Interim version every six month, with a major LTS release every 24 month.
Generally speaking, focus on stability and support rather than development software is a good idea for server.
Here are the main links you need to install an Ubuntu server operating system:
Ubuntu 20.04 Server Page - Choose Manual install and choose your architecture version
Direct official download - Ubuntu 20.04 LTS Server ISO for AMD64 architecture
Official torrent link to download Ubuntu 20.04 LTS - AMD64 architecture
Note: If you want to run your server on a Raspberry Pi, the best entry point is https://mazizone.net