Do I need to already have a website?

No. Pono includes a fully featured website/portal out of the box.

We'll initially set your Pono system up on our ponoapp.com domain. Your site will be something like mysite.ponoapp.com.

You can connect your own domain anytime. You can then run Pono in a subdomain and keep your existing frontend, or you can use Pono to power your entire website.