To get the Client ID and the Secret Key from Paypal you need a Paypal account.

1. Open the following website and click on "Log into Dashboard"

Create a Developer (Sandbox) account

2. After you have logged in, createSandbox Business Account to be able to make test payments later.

Create Paypal App

3. If you have created a sandbox account, click on "My Apps and Credentials" in the menu on the left and then on "Create App"

Paypal App Client ID and Secret Key

5. As soon as the app is created, the details of the app you just created will open. You should now see the Client ID and an option (Show) below to see the Secret Key.

Then add the ID to and you should be good to go