The very first things you need to learn and get comfortable with:
- unix (file system, permissions, processes, services, logs, networking, etc…)
- ssh & reverse tunneling
- nano/vim
- python
- git
Then you could move on to more specific topics:
- containerization (e.g. docker)
- configuration management (e.g. ansible)
- continuous deployment (e.g. jenkins)
- monitoring (e.g. prometheus/grafana)
- etc…
