This project will help you get started. It has sample code to allow a store to install your app(free) and get a token for the store. This token will allow you to make API calls to Shopify.
This project scope is: read_products,write_products but you can change this for your app.
You must add a .env file to your project to save your app settings. Make sure you don't commit this file to your source code
I'm using ngrok.com to tunnel traffic from the web to my localhost app.
Videos about this app are on youtube.com/user/videocode2
Make sure you install https://www.npmjs.com/package/dotenv for the .env file to work.