When creating responsive emails it's important to use the Structures and Blocks for each section of your email templates.
Structures are the outermost layout which allows you to define the number of columns for this section of the email. An email can have many sections and each section:
Blocks are the elements you will put into the column for example images, text, buttons etc:
See here a short video that explains how to use the Structures and Blocks of the email builder: