How to not overlap nav bar over remaining page

HTML-CSS
1

As you can see my nav bar is overlapping the main section of the documentation page. Is there a specific property I should know about to solve this problem or will I use the margin to align it with the nav bar. By the way I just started an hour ago so I know i will have a lot to work on!

<!doctype html>
<html>
<head>
	<link rel="stylesheet" type="text/css" href="stylesheet.css">
<title>PHP</title>
</head>

<body>
<nav id="nav-bar">
	<br>
	<ul>
	<header>
	<li>PHP Documentation</li>
    </header>
	<br>
	<li id="nav-link">Introduction</li>
	<li id="nav-link">PHP forms</li>
	<li id="nav-link">PHP Advanced</li>
	<li id="nav-link">PHP MySql Database</li>
	<li id="nav-link">PHP XML</li>
	<li id="nav-link">PHP Ajax</li>
	<li id="nav-link">PHP Examples</li>
	<li id="nav-link">Refrence</li>
</ul>
</nav>	

<main>
<section class="main-section">
<header>Introduction</header>
<article>
	<p>PHP is a server scripting language, and a powerful tool for making dynamic and interactive Web pages.

PHP is a widely-used, free, and efficient alternative to competitors such as Microsoft's ASP.</p>
	</article>
</section>

<section class="main-section">
<header>PHP Forms</header>
<article>
	<p>The PHP superglobals $_GET and $_POST are used to collect form-data.</p>
	<p>The example below displays a simple HTML form with two input fields and a submit button:

Example</p>
<code>
html
body

Welcome <?php echo $_POST["name"]; ?><br>
Your email address is: <?php echo $_POST["email"]; ?>

body
html
</code>
	</article>
</section>

<section class="main-section">
<header>PHP Advanced</header>
<article>
	</article>
</section>

<section class="main-section">
<header>PHP Mysql Database</header>
 <article>
 	</article>
</section>

<section class="main-section">
<header>PHP XML</header>
<article>
	</article>
</section>

<section class="main-section">
<header>PHP Ajax</header>
 <article>
 	</article>
</section>

<section class="main-section">
<header>PHP Examples</header>
<article>
	</article>
</section>

<section class="main-section">
<header>Refrence</header>
<article>
	</article>
</section>

</main>
</body>
</html>

*{
	margin:0;

}

body{
	background-color:#eef5f1;
}

#nav-bar{
  
  background-color:#e5f1ea;
  flex-direction: column;
  width:15%;
  height:100%;
  position:fixed;
  

}
header{
	margin-right: 20px;
}
li{
	list-style:none;
}


.main-section{
	margin-left:100px;
}
2

Can you add a link to your demo?