Your hosting plan will automatically renew every billing cycle (normally month-to-month).
To ensure renewal, all you need to do is pay for the hosting starting when your invoice is created (5 days before the due date) and within the 10 day grace period (10 days after the due date). If you have been with HostGator for more than 90 days, you may also overpay your invoice. The excess amount will be applied to your future invoices (just like paying in advance).
You can view and pay your invoices in our billing center at http://portal.hostgator.com. For more specific instructions, please read our related articles linked below.