1. First, you will need to setup a business account with PayPal. (Technically you can test with a free account first).
  2. In the PayPal Developer Dashboard (https://developer.paypal.com) you will need to setup a new App to handle payments.
  3. You should now see a sandbox credentials for the app, as shown in the screenshot below. This is located in the My Apps & Credentials menu.
  4. After testing with the sandbox credentials, you can get the Live credentials by clicking on Live in the top right, as marked on the screenshot below





Here is a reference to the PayPal documentation which describes these instructions in more detail.

https://developer.paypal.com/docs/checkout/integrate/#5-go-live