Hey,
you need to start here: TYPO3 site package tutorial — Site Package Tutorial 12.4 documentation
In th text above the cards is a link to a github repository for an example site package. You can use this example or you have to create your own. GitHub - TYPO3-Documentation/site_package: The code for TYPO3CMS-Tutorial-SitePackage
In the summary section Next Steps — Site Package Tutorial 12.4 documentation there is another possibilty described. With using the “Site package builder” you can create your with just a few clicks. Follow the link: Kickstart your TYPO3 template development - get.typo3.org
I hope that helps you.