Media query for small screens

Please i need assistance for my media query. screen not responsive for small screens. what am i getting wrong?

How do you want certain elements to behave when the screen size is 400px (a mobile device)?

So in the case where the whole element in the screen is not responsive to small screen where will i place the media query?

Can you be specific about which element? What do you want to happen to the specific element when the screen width is 400px? Unless you tell us exactly how you want things to look, we would only be guessing as to the look you want. Do you have a sketch or wireframe showing how you would like the various elements to render when the screen width is 400px?