Used html and css to make an responsive product screen

This project as the first time in my life that a tried to use or even heard about responsivity in general, so making something responsive in general as really hard for me, 80% of my time building the project i was lost trying to know why in the f##### world the thing that a did 2 minutes ago stopped working... i almost gave up at some point. And i would love to know tips to make responsivity more easy, because I'm a little traumatized after this experience and it's a really easy project. (And sorry for the bad english, first time a tried to write something in another language)