You can’t solve a problem without having a basic understanding about it. Likewise, you cannot create a theme without understanding its basic structure. You need to fully understand the anatomy of the WordPress theme.
A standard WordPress theme consists of a header, sidebar(s), a loop, and a footer section.
Yoast has a brilliant infographic on the same, covering all the core theme files. You can check it out over here.
For further reading, check out the Anatomy of a WordPress Theme by WP Engine & Understanding the Anatomy of a Basic WordPress Theme by Theme Spotlight.