Here are some helpful resources that can help you learn the basics of Shopify theme development:
Shopify Design Tutorials
Informative videos by Shopify's @keirwhitaker on making themes. These videos cover specific questions or tasks you might want to undertake when building or customizing a theme.
Timber is a front-end framework which makes building Shopify themes quick and easy. It includes all of the required templates and Liquid tags as well as a responsive CSS framework to quickly apply your own styles.
Skeleton is another great starting point for theme designers. It includes inline, detailed comments to help you better understand each template.
You can download Skeleton through its GitHub page here.
Shopify Cheat Sheet
The Shopify Cheat Sheet is an interactive reference for the Liquid templating language used to build Shopify themes.
You can explore the Shopify Cheat Sheet here.