Install 5.7 into a different directory, and create a separate database for it. I find it quicker to develop new sites on my local computer before uploading them to the server.

I haven't built one on my local computer before. How would I do that? Do I still need to download the newest concrete5 version and then the theme I'm buying from concrete5?

If you want to develop locally, consider installing XAMPP, which is a full stack that includes Apache, MariaDB (MySQL compatible database), and PHP, three core requirements of concrete5. If you are on Windows, you could use WAMP as an alternative. WAMP has MySQL as a component of it's stack.

Outside of local development, you could also spin up a concrete5 instance at one of many hosting providers that offer one-click deployment, Bluehost being one (I don't advocate for "commodity" hosting providers, but if you've not installed concrete5 in the past, this is a simple, low-cost option).

Hope that helps.

Thanks.

As blueatlas said. I'm using XAMPP on Windows.

Yes, you have to download concrete5.

The theme could be a problem. I've never managed to get my XAMPP concrete5 to connect to the concrete5 community (despite hours of trying), so I probably couldn't install non-free themes or add-ons. I gather others have managed to connect local installations to the community, so YMMV.

Thanks.