If you have an open-source shopping cart (magento, oscommerce, opencart, prestashop, etc...) and decide to accept credit cards through them, what else is involved when setting up a payment gateway?

Say I want to use authorize.net, how does my OS shopping cart transfer information to these payment gateways securely?

I know there are a lot of requirements to being PA-DSS certified, so what kind of things does the merchant have to do in order to be secure?

If I have someone give credit card information through a form on OS Commerce, for example, how does that information become SSL encrypted and pass to authorize.net securely for payment? Do payment gateways take care of this?

What step am I missing?