In WooCommerce 2.2, the Simplify Commerce gateway was added to WooCommerce. This is huge news, because Simplify Commerce is a really user-friendly gateway and it processes credit cards without taking you to another site (unlike PayPal). Simplify Commerce is pretty easy to set up and doesn't require a merchant account as some older processors do. If you're just getting started, Simplify Commerce is an excellent payment gateway to use.
Note
Simplify Commerce is only available to merchants in the United States. If your store location is set to another country, you won't see the gateway.
An SSL certificate is required to process actual credit cards. Contact your host about setting one up on your site. You don't need an SSL certificate for testing.
There are some other technical hardware requirements listed at http://docs.woothemes.com/document/simplify-commerce/.