Available OSs on a dedicated server

You can select FreeBSD or one of the Linux distribution: 
FreeBSD: free Unix-like operating system for a variety of platforms that focuses on features, speed, and stability.

Debian: one of the oldest operating systems based on the Linux kernel. It is also one of the most popular Linux distributions, and many other distributions have been created from its codebase.

Ubuntu: Linux distribution based on Debian and composed mostly of free and open-source software. It is one of the most popular and beginner-friendly Linux distributions.