How to adjust product 's template to the formats


I’m having trouble adjusting my product’s template design to other formats, on website looks all great, but when I go to tablet or phone it’s all just a mess.
Also my menu nav disappears on these other formats.
Almost everything’s margin is on automatic but elements on top of each other.
Hope someone can help me, thanks!

Here is my site Read-Only: [LINK][1]