user registration

Hey all, first time poster here. I have a few WP blogs self hosted, I would like some help or knowledge about adding user registration, i set blog to only accept comments from subscribers but how do they subscribe? I applogize if this has been asked before but am curious, do thy register on my site ie. widget/plugin? or is it external to the site itself? is tehre a database created in my databases fior the user info or does it need to be self created? Thanks in advance for nay help 🙂