Responsive css

Nice work with challenge. There are some shortcomings.

1.) First, answer to your question, use media query for the different sizes. One way I use is first develop the mobile design, then apply media query for tablet design and then finally, another media query for desktop design.

2.) Your code is not semantic, try not to use lot of div tags and use more semantic tags. (You can search about semantic tags if you don't know about them.)

3.) I can see in your code that you have media query at the middle of code, that's not a good practice. Always put your media queries at the end of your CSS file. The reason for that is CSS will override media query if it is not at the end of CSS file.

That's all for me, keep coding.