To renew your domain you login at your registrar and follow their instructions for payment.
Do make a note to yourself for when the domain is going to expire and make sure that you have paid before the deadline and you should be ok.
You don't have wait until the expiry date to renew, and I'd recommend renewing well in advance (like today). That will make sure you have time to sort out credit card problems or other glitches.
One problem we hear about sometimes is that someone changes their email address but forgets to update their WHOIS information. That means they don't receive the registrar's reminder emails when it's time to renew, and the registration lapses through the user's own negligence. Not fun!