You will need a very good understanding of:
Failing all of the above, simply find a WP Theme (free or premium) that you like.
In addition to the excellent links provided by Nancy, I would add that if you are starting to create a template for Wordpress, I strongly advise you to use a starting base (Wordpress Starter Theme), as well as a methodology of approach, not to mention a good understanding of how Wordpress loads and organizes its structure.
On the structure side, you could get closer to this old article which, although dated, allows you to understand the interweaving of modules and their interpellations https://www.rarst.net/wordpress/wordpress-core-load/
for the approach, Francis also wrote a great article in 23 steps... it's very old, but it will give you some basic approaches (sorry it's in french but between deepl and translator you should find a solution) http://www.fran6art.com/wordpress/creez-votre-theme-wordpress-de-a-a-z/