To get the Client ID and the Secret Key from Paypal you need a Paypal account.
1. Open the following website https://developer.paypal.com and click on "Log into Dashboard".
Create Paypal App
2. Click on "My Apps and Credentials" in the menu on the left and then on "Create App"
Ensure that you are on a 'Live' app
3. Give your Paypal app a name (e.g. Osu 😊) After you have finished, click on "Create App".
Paypal App Client ID and Secret Key
4. 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.
5. To switch between Live and Sandbox (if relevant) click first on "My Apps & Credentials" in the left main menu then click on Live or Sandbox and then on the desired app you want to know the Client and Secret keys. Note that Sandbox and Live have different Client IDs and Secret Keys. Make sure you copy the Client ID and Secret Key from the Live app.