<div id="container">

 <div id="main"> Main content here </div>
 <aside> Aside content here </aside>

</div>

body {
  margin: 50px auto;
  font-family: "Lato";
  font-weight: 300;
  width: 600px;
}

#container {
  width: 600px;
  height: 400px; /* just for demo */

  background: #e3e3e3;
  margin: auto;

  display: flex;
}

#main {
  background: yellow;
  width: 400px;
}

aside {
  background: red;
  flex: 1;
}