Depends on what you want to self-host. In general, I would advise against self-hosting anything before you familiarise yourself with the basics of *nix, networking and cyber security.

You at least need to know enough to make sure that whatever you host is only available within your local network and is inaccessible from the outside.

Once that’s ensured, go nuts, experiment, learn, evolve.

In terms of how to start, really depends on your budget, what hardware you can spare, how much space you have at your place etc.

For the most basic playground it’s enough to have a raspberry pi or similar, or a very old laptop / desktop computer.

For something more swanky you can get old Dell servers (e.g. R420) online for around 100$ or so. They are quite power hungry though. Or you can get yourself a NUC and use that.

If all of this sounds like too much work, just get yourself QNAP / Synology NAS and see what it can do for you (it is way more limited in terms of options, but easier to setup and you can still have your Plex / file sharing / docker containers).