How would I know when someone submit a payment?

I would we know someone has paid if we have just a plain paypal account?

Well, you should receive an email for successful transactions.  Email from Jotform and email from Paypal. 

Note that when someone does not complete the payment, it will be logged as incomplete. You can check incomplete payments to your form by following this guide: http://www.jotform.com/help/564-how-to-view-incomplete-payments-of-your-payment-forms/. 

Thank you!