Go to https://www.drupal.org/project/paypal_payment and download the module for paypal.
Enable paypal,PayPal WPP and PayPal WPS.And save configurations.
After this check if Payment Method and Payment UI is enabled if not enable it and save configuration. If they are not enabled we cant get the Payment drop down for next step.
Now go to Store> Configuration > Payment method
go to ->Enable payment method: PayPal WPS -click edit
Enter your Paypal email address
Log notifications with the full IPN during validation and processing (used for debugging).
tick check box -Show a message on the checkout form when PayPal WPS is selected telling the customer to “Continue with checkout
to complete payment via PayPal.”
and then save