Just jump right in.

The 2 most popular - Bootstrap and Foundation (the 2 biggest responsive frameworks) are easy to use and well documented. It comes down to a matter of preference and need. Bootstrap tends to be more wildly supported but heavier while Foundation tends to be lighter, but not as often used. Both are configurable and customizable to change grid sizes, button colors, etc...

Hey mate, here's a fantastic article from Smashing Magazine: smashingmagazine.com/2011/01/guidelines-for-respo… It runs you through the most important aspect, the why of Responsive Web Design!

Here's a huge BootStrap tutorial / resource: tutorialrepublic.com/twitter-bootstrap-tutorial

Are you from Javascript background ? If so you need to learn SASS or LESS because its extensively used for adjusting the screen dimensions during runtime. Also Media Queries is used for that purpose. Please check Twitter Bootstrap section on Grids.

A list apart to the rescue :)

alistapart.com/article/responsive-web-design

Are you interested in design or the coding?

Because the RWD is a very loose world where you have adaptive, responsive and fluid designs. And are you interested in the grid system?

And yeah, go with Sass. That's much more important to learn.