One cannot come up with a solution for a problem one cannot understand. Likewise, you cannot create a theme without understanding its basic structure. You need to fully understand the anatomy and architecture of the WordPress theme.
A standard WordPress theme consists of a header, sidebar(s), the loop, and the footer section.
Yoast has got a brilliant infographic on the same covering all the core theme files. You can check it out over here. Additionally, you can watch the below video –
For further reading, check out the Anatomy of a WordPress theme by Layout from Flywheel.