.header {
/* styles */
position: fixed;
margin-top: 50px; /* or whatever */
}<!DOCTYPE html>
<html>
<head>
<title>Testing My HTML and CSS</title>
<style>
* {
padding: 0;
margin: 0;
}
.header {
background-color: black;
height: 100px;
width: 100%;
}
.header h1 {
margin-top: 0;
text-align: center;
color: white;
height: 100px;
width: 100%;
}
.sidebar {
background-color: #ebebeb;
position: absolute;
width: 200px;
height: 100%;
}
</style>
</head>
<body>
<div class="header">
<h1>Hello, World!</h1>
<div class="sidebar">
</div>
<div class="content">
</div>
<div class="footer">
</div>
</body>
</html>