How to add PayPal credentials?

Go to PAYPAL/ Paypal Config on your Admin panel. If you have a standard PayPal account you can add a widget for PayPal button to checkout template and to the quote template and you only need to fill in the email field.

In case you have a Pro account (only available for USA, Canada and Australia) you can insert the direct payment form on checkout using this widget . For this you would need to feel in all fields.