@charset "utf-8";
/* CSS Document */
body {
margin: 0;
padding: 0;
display: block;
background-color: darkgrey;
}
.logo{
width: 100%;
background-color: midnightblue;
position: fixed;
height: auto;
text-align: center;
}
.nav {
float: left;
position: fixed;
width: 100%;
height: auto;
background-color: black;
}
.nav a{
display: inline;
text-decoration: none;
color: antiquewhite;
}
.article{
width: 70%;
height: auto;
align-content: center;
background-color: antiquewhite;
margin: 20px;
}
Let's say, this is what you're tyring to do.

The entire code would look like this. I'm using embedded CSS here for expediency.
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Document Title</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<style>
body {
margin: 0;
padding: 0;
background: darkgrey;
}
header {
background: midnightblue;
color: white;
}
header img { width: 100%; }
.flex-grid { display: flex; }
.col { flex: 1; }
nav {
text-align: center;
width: 80%;
margin: 0 auto;
}
nav ul { list-style: none; }
nav ul li { display: inline; }
nav ul li a {
text-decoration: none;
padding: 5%;
color: antiquewhite;
}
nav ul li a:hover, nav ul li a:active, nav ul lia:focus {
color: midnightblue;
text-decoration: underline
}
article {
background: antiquewhite;
padding: 5%;
}
footer { text-align: center }
</style>
</head>
<body>
<header class="flex-grid">
<div class="col"><img src="https://placeimg.com/300/150" alt="my logo"></div>
<div class="col">
<h1>Hello World!</h1>
<h2>Welcome to my awesome website</h2>
</div>
</header>
<nav>
<ul class="nav">
<li><a href="#">HOME</a></li>
<li><a href="#">ABOUT</a></li>
<li><a href="#">CONTACT</a></li>
</ul>
</nav>
<article>
<h2>Heading 3</h2>
<p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Dolore nobis, itaque est dolor! Sit provident sapiente corrupti obcaecati blanditiis, iste at, ullam quasi quam eligendi sunt! Ea, dolorem, iure! Placeat.</p>
</article>
<footer>
<p>Footer text</p>
<small>XYZ Company © 2018</small> </footer>
</body>
</html>