Learn how to use Shopify
Where should you start?
Your best bet to start is the official Shopify from scratch tutorial. This will help you build a theme from the begining walking you through liquid, loops, templates and more.
The best way to create a development shop for building your theme is to become a Shopify Partner and create a "development shop".This dev shop is effectively a sandbox store that is crippled in a few ways like not being able to accept orders and a limited number of SKUs.
Essential Shopify theme resources
Shopify Cheat Sheet - This cheat sheet gives you a list of all the Shopify variables you can use in your store. It has examples for most of the template variables.
Shopify skeleton theme - This is perfect if you want a boilerplate/sample theme to start hacking on. It contains most of the major features you want in a store and is a great starting point for many new theme authors. Also consider downloading one of Shopify's free themes as a starting point.
Desktop Theme Editor - The desktop theme editor let's you edit your Shopify store remotely from your desktop. You can save a file locally and it will automatically sync with the live version of your site.
If you're using git and multiple people are using your store I'd recommend using the Shopify Theme Gem. This is a command line interface for syncing your live store with your desktop assets.
Official Theme Docs - on this page you have everything a Shopify theme designer needs. In the sidebar there is a list of all the liquid templates and variables. It's the most comprehensive guide on theme designing.
Theme from scratch tutorial - another great theme building tutorial from the beginning by Tetchi. As of this writing he is still completing it, but hopefully it will be complete by time you complete it.
Other helpful Shopify resources
Shopify forums - a great resource for searching questions designers have already asked and solved.
Shopify and you - an essential primer to getting started with your Shopify store. A great read for clients and designers alike. It will walk you through all the basics giving you tips along the way to make your store as successful as possible. The author also wrote an must read intro to Google Analytics for ecommerce stores.