Once you sign up, you should expect to receive your bill within a few days, soon after which it will be due. If you are on AutoPay, your payment method will be charged on the due date. In that first bill, you will be paying for your current month of service (the days since you signed up through the remaining days in a 30-day period).
The day that your bill is due will be the same each month. If you incur any additional usage charges (such as International calling or roaming), the charges will appear on your next month’s bill. The same applies for any late charges incurred.