Ya got three options.

Option A is to create your own certificate that is self-signed. You will then have to load the certificate into any client you want to use. Easier than people realize, just a couple terminal commands. Give this a go if you want to learn how they work.

Option B is to generate a certificate with Let's Encrypt via an application like certbot. I suggest you use a DNS challenge to create a wildcard certificate.

Option C is to buy a certificate from your DNS provider aka something like cloudflare.

IMO the best is Option B. Takes a bit to figure it out but its free and rotates automatically which I like.

I like helping and fixing stuff, if you'd like to know anything just ask :D

Kleopatra