Here is my css:

body {
}
#sidebar {
background-color:lightgray;
width:200px;
position:fixed;
height:1000px;
float:left;
}

li {
list-style-type:none;
}
a {
display:block;
background-color:white;
width:150px;
text-align:center;
}

And here is my markup:

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
<link href="StyleSheet1.css" rel="stylesheet" />
</head>
<body>
<form id="form1" runat="server">
<header>Header</header>
<div id="sidebar">
<ul>
<li>
<a href="WebForm1.aspx" class="btn">Link1</a>

</li>
<li>
<a href="WebForm1.aspx" class="btn">Link2</a>

</li>



</ul>
</div>
<div>
<asp:ContentPlaceHolder ID="ContentPlaceHolderMain" runat="server">
</asp:ContentPlaceHolder>
</div>
</form>
</body>
</html>

Currently, it's not centered. It's currently to the right.