After you install the paypal payment gateway, you will see this settings:
USD Conversion - If you enable this option, the payment amount will be automatically converted to USD. For example, if your system currency is set to Philippine Peso (PHP), it will automatically convert to USD in paypal's payment page. This is specifically useful if your system currency is not supported by paypal because using an unsupported currency can cause issues with the payment gateway. Supported currencies for paypal are here: https://developer.paypal.com/docs/reports/reference/paypal-supported-currencies
Sandbox Mode - Enable this to test the payment gateway without processing real transactions. Make sure to disable this when you are ready to go live.
Client ID - Your paypal client ID.
Secret Key - Your paypal secret key.