There are many resources available that can help you gain proficiency in template development. WP Codex, the official section of WordPress.org provides extensive documentation on the most popular CMS in the world. And, among all other data, information related to theme development is also offered.
Other possibly useful resources are various online courses from platforms like Udemy, Coursera, or Codecademy. You could also consider reading WP-related books or attending local WordPress Meetups or WordCamps.