The best thing about using an invoice system is that you can track your time against the amount you're getting paid for each task. In order to run a successful business, you want your hourly rate to be profitable so you're not losing time and money. Another benefit of using an invoicing system is that you'll never save over an invoice and lose the work you've calculated for another client.
Next, you need to specify your payment options — whether you prefer to be paid with cash, a check, a credit card or a service like PayPal. Some companies will offer direct deposit, if you do regular work for them, but more than likely you'll have to send an invoice to request payment every time you complete a project.