I’m so glad you are interested in Responsive Design. It really does help to make the web a better place.
Responsive Design is really a state of mind, and you want to be thinking about it right at the beginning of your project.
May I suggest this video? https://www.udacity.com/course/responsive-web-design-fundamentals–ud893
This video is what jump started me with Responsive Design. I hope it does the same for you!