If you have a credit card on file and set to auto-renew then there is nothing to do except to ensure your credit card is still active. Alternatively, you can register for up to 10 years so you do not need to worry about price hike or anything during that time frame. About 1 to 2 months before your domain name expires, they will send you an email remind every week until the domain name expires to renew. You can renew within the godaddy control panel.
if your domain in godaddy, only click "renew" link.. if it is in other registrar, you must transfer it to godaddy first, then they will renew your domain free for 1 year..
The price for renewals is the anual price that you see for each extension (usually without the discounts).
Depends on the extention. The best thing would be to go to Godaddy, go to the checkout with the name & you'll see the exact renewal that it'd cost you.