What kind of operating system admin dist you want to use? Debian/Ubuntu Arch or CentOS/Fedora etc? You can Explore it through DO ( Digital Ocean Explore Navigation) or go to this Link:
You can learn it step by step. Select the operating system you run for a long term and focused with your operating system. And it helps if you visit the official website of the operating system that becomes your choice to get more information. And I think, DigitalOcean already has many articles of information that make it easier for users to recognize and run the operating system and applications.