In order to begin building with PayPal APIs, you have to create an application to obtain a client ID and secret.
Go to https://developer.paypal.com/developer/applications/, sign in, and click on "Create App", as shown below:
Next, enter an application name, select the sandbox testing account that you want to use (if it's a new account, leave the default value), and click "Create App".
Once the application is created, you will be provided with your sandbox and live client ID and secret, which will look similar to the following:
These credentials are what you will use when making requests to PayPal APIs in order to authenticate your application and make requests.