How to implement fixed sidebar correctly?

Use the content div as your container for the page.

 .sidebar {
     position: fixed;
     width: 200px;
     height: 400px;
     background: #000;
 }
 .content {
     margin-left: 200px;
     height: 500px;
     width: auto;
     position: relative;
     background: #f00;
     overflow: auto;
     z-index: 1;
 }
 .info {
     width: 1440px;
     height: 300px;
     position: relative;
     background: #f55;
 }

<div class="sidebar"></div>
<div class="content">
    <div class="info"></div>
</div>

Your content will need to be the container to put the page in. The values here are my test to see if I am correct in this. If your width and height exceeds the values you set for content, the scroll bars will appear.

Have a fiddle: http://jsfiddle.net/djwave28/JZ52u/

---

edit: responsive sidebar

To have a responsive fixed sidebar, simply add a media-query.

Example:

@media (min-width:600px) {
   .sidebar {
     width: 250px;
   }
   .content {
     margin-left: 250px;
   }
 }

Here's another fiddle: http://jsfiddle.net/djwave28/JZ52u/363/

Here is an alternative: http://jsfiddle.net/BoyWonder/8mVQX/embedded/result/

body{
    padding-left:200px;
    margin:0;
}
div#sidebar{
   position:fixed;
   height:100%;
   width:200px; 
   top:0;
   left:0;
   background:grey; 
}
div#content{
    background:black;
    width:100%;
    height:1600px;
}

Here is another alternative by using only two CSS lines

.sidebar { position: sticky; top: 0; }

and the credit goes to this post.

You can also experiment with the code over here.