How to turn a magento theme into a responsive ?

Hey, turning any theme to become resonsive is done by including a responsive css framework such as bootstrap, and then you have to design from the wider view to narrower one, you have the elements, but you need to add the appropriate classes for your responsive css framework, or you can compile something using less or SASS. either way it all comes down to using media queries, and planing how your theme will look on various screen sizes. 

You can customize your Theme CSS file with Media Quries to work well on different screen sizes , or longer path is write your theme from scratch 

Responsive is just old Elastic % based design so dont take it that much on your mind only thing is with media quries you are overwriting your css style based on Screen size 

Hi, Magento theme convert as responsive include responsive media query and using grid or fluid responsive css and all magento page and block class using media query class. Starting to home page, landing page and details page. Its better to include responsive ecommere template and covert you theme according this template. It may help you.

I would rather recommend magic liquidizer to turn your magento website to become responsive without hard coding. Its a magento extension, so expect to plugin and play