You can select a PayPal account or create a new PayPal account
By checking "Use Live PayPal Server" checkbox, all purchases from the store will be real money transactions, uncheck this checkbox to use PayPal sendbox server (test server), no real money transfered.
To obtaining your PayPal API signature credentials, please refer Creating and Managing Classic API Credentials
PS: please clone the template-store site and call it "store" for your online store