To start off you need to install Shopify App CLI. Why use Shopify App CLI? Because it helps you build apps faster. It takes care of the boilerplate code during initial setup, so you have a template to base your app on. In short, a quick and easy setup to start developing apps.
There are differences between operating systems on how to get this setup. This link will provide you with instructions on each: https://shopify.github.io/shopify-app-cli/getting-started/install/
My machine is a Mac, so the installation is as simple as installing Homebrew through this link: https://brew.sh/ . Then installing the Shopify CLI through two brew commands brew tap shopify/shopify first, and then brew install shopify-cli.
Use command shopify version to verify it has been installed. A version number should come up. For example: 1.0.0.
If you need to update shopify-cli just use these commands: brew update and brew upgrade shopify-cli.
To view all the different commands shopify-cli has to offer use command shopify help. If you want to learn more about a specific command use shopify help [command], replacing "[command]" with the command of interest.
We will also need to download Node.js to your machine. Head over to https://nodejs.org/en/, download, and install the Node package that is "Recommended for most users" if you don't already have it. Use command node -v to view the node version number and to verify it has been installed.
One last thing before we get started, you will need a Shopify Partners account and a Shopify development store to deploy your test app. Follow along this blog post to get setup: https://joepichardo.com/blogs/shopify-liquid-for-beginners/free-shopify-store-for-liquid-theme-development