Zen Subtheming on Drupal 8 Question

I’m a beginner to Drupal and just started using Drupal 8. I have a question for subtheming with zen, in what location do I start writing my css/html for my website? Is there a specific file where I start to write all the css to or do I need to create one of my own? If I create a new file, do I need to add any other commands to another file to make sure it is executed when loading the theme?

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