How to use media query in CSS

<meta name="viewport" content="width=device-width, initial-scale=1.0">

@media only screen and (max-width: 600px) {
  div.container {
    margin: 0 auto;
      width: 70%;
      margin-top: 60%;
    }     
}

body {
  background-image: url("../images/wallpaper.png");
  background-position: top right;
  background-attachment: fixed;
  background-size: cover;
}