When you're on developer.paypal.com you will Log in using your PayPal credentials. Next you'll navigate to My Apps & Credentials, where you will create an app for what you are trying to accomplish (in your case adding Payments into the booked scheduler). Scroll to REST API and create a REST app (you can name this anything you need to help you identify this app from others). Once completed, you'll see your client ID and secret.
Note: You have to toggle in the right corner for live vs. sandbox, as it defaults to sandbox at first.