The software itself is indeed free. However, you need a means of hosting it so others can access it (after all, what good is a forum that only you have access to).
If you already have access to a server, then yes, there is a ‘free’ option.
If you do not have access to a server, you will need one that can run something called docker.
DigitalOcean’s $10.00 a month plan can host small communities. Someone can walk you through the installation process or you can do the additional $99 fee that Discourse offers.
Otherwise, you can have DiscourseHosting host and install it for 20.00 a month, or have Discourse host it and get far more direct support.
Otherwise, you can always jump into an existing community, help out, and help built out that community. I suppose the question at hand is, what exactly do you want to do, and why do you want to do it?