If you’re looking to build a theme from the ground up, you’ll want to look into a few things.

First – setting up a local server. If you don’t have experience with this, DesktopServer is a quick way to get started. MAMP is also a good solution. Both have free options.

https://serverpress.com/get-desktopserver/
https://www.mamp.info/en/

Next you’ll want to read up on how themes work and what files are invovled. Check out the theme handbook.

https://developer.wordpress.org/themes/getting-started/