First, check that Node.js (v12 or later) and npm are installed:
node -v
npm -v dependencies
If they are not installed, you can use homebrew to install them:
brew install node
Next, install dependencies that are requiered to run the website locally using npm:
npm install
Finally, serve the site locally:
npx @11ty/eleventy --serve
The site will be at http://localhost:8080/.