This introduction course delivers basic instructions about Responsive Designs theory and Bootstrap applications.
Personally, I did not recommend that your start your learning using Frameworks, cause it’s hard to understand what the code is doing. First, study the language from the basics to not get confused.
I don’t use Bootstrap, but if your interest about how to starting to create websites, it’s a good option to facilitate your work. In course we have examples of implementations of how to use Bootstrap to create Responsive Websites.