I tried docker and stopped shortly after, because it was adding a layer of complexity I was not prepared for at the beginning. So I started with services I could run on bare metal with an OS like Ubuntu. For this a basic website for clients could be a good starter because it might only need a reverse proxy with php and a database. BUT this already opens questions around how secure is your server, which would be the first topic I would focus on, especially when hosting stuff for clients. Because if something happens to their data, you‘re responsible for it.

If it‘s just a static page like a portfolio, simple things like restricting access with firewalls ACLs and other basic server configurations might already be enough.

For hosting email, this topic still hasn‘t come to a mence for me, because it sounds like a lot of time and effort that goes into maintenance so I keep looking for hosted services with a good privacy approach.

What resources are you thinking of (there are many). Again, the best way of approaching it is take something with little dependencies and components that are well documented and easy to learn, so you can focus on how to monitor the host, and make sure you can administrate it fast and efficiently.

Later on you can add more complex services and learn about the new components.