I need to have a checkout page that allows the user to insert his data in one step. So all the sections (billing, shipping, shipping method, payment method, review order, place order button) are in the same page displayed at the same time. How can I do this?
Go to http://www.magentocommerce.com/magento-connect and type 'checkout' in the Search box.
You'll find there are dozens of extensions. Some are free.
But whichever one your choose, be aware that there's still a lot of filling out to do, albeit it on just the one page.
This often involves the use of clumsy drop-downs. And if PayPal is involved some checkout pages take you away from your site and you may lose your customer in the final step: payment!
The best plugin I know that makes the UX of any checkout page really smooth is Braintree's latest "drop-in" UI, known as "v.zero" and we do a Magento extension for it.