Setting up a Tor hidden service is pretty straightforward. In a simple case one needs to follow Tor Project's instructions. Also, it is a good idea to take a look into Riseup's Best Practices.

So, now my website is also available as a Tor hidden service at the following address:

xbetver6rggmt2vdxlcwzjqjrhurklsevvnzzctvrwbzd6knukmw2iid.onion

I've added the following settings to my /etc/tor/torrc:

HiddenServiceDir /var/lib/tor/hidden_service/
HiddenServicePort 80 127.0.0.1:80
HiddenServicePort 443 127.0.0.1:443

Also, I had to switch off Apache's mod_status, to prevent statistics leakage through Tor hidden service. To switch it off one need to run a2dismod status (it is enabled by default in Debian). Anyway I don't need it.