Well, as everything, it is safe is you do it right. If you are just starting, you could try running some services in LAN, or if you don't have the hardware, a cheap VPS with VPN or some other tunneling. That way, you're not exposing your data to the wider internet.

Of course, you should keep backups of everything you deem important, and once you start feeling more comfortable, you may try opening up your services with a reverse proxy (caddy, nginx) and some authentication (authelia, authentik). You can try hosting simple things, things that expect to be open to the internet, like a game server.

If you're limited by your hardware, you can start planning to buy something bigger. But I don't recommend it to just get started; you can do a lot of learning with basic hardware (and the prices are crazy right now too).

Edit: seconded what ghost_laptop said, an e-mail server is kind of the hardest there is for self-hosting. There are solutions like mailcow, but even that is very hard to set up.