1.) Login or Signup for PayPal https://developer.paypal.com

2.) Go to "Apps & Credentials" (1) and create new app "Create App" (2) or select app from list "REST API apps" (3)

3.) Copy Client ID (1) and copy Secret key (2) to you admin panel, section "Payments Settings", to PayPal Client ID and PayPal Secret Key

4.) Then in your admin panel, section "Payments Settings", you can disable or enable the PayPal function, as well as configure additional settings.