In terms of authenticating with Shopify Private Apps, you can get API Keys (i.e. API Key and Secret Key) by using the Partner dashboard or your Shopify admin panel.
#1: Getting keys using Shopify admin panel
Log into your Shopify admin panel, go to Apps Menu.
Scroll down the page and click on Develop apps for your store.
Shopify-API-Key
Click on Create an App and then enter your App name.
Click on API credentials then scroll down, in the Access tokens area, click Configure Admin API access.
In this area, tick all Permissions “Read_….” and “Write_…” (if any).
Click Save App, at the bottom of the page.
Choose API credentials and you will see API Key and Secret Key (i.e. API Key and Password)