Here you go:
<html>
<head>
<title>Cols</title>
<style>
#left {
width: 200px;
float: left;
}
#right {
margin-left: 200px;
/* Change this to whatever the width of your left column is*/
}
.clear {
clear: both;
}
</style>
</head>
<body>
<div id="container">
<div id="left">
Hello
</div>
<div id="right">
<div style="background-color: red; height: 10px;">Hello</div>
</div>
<div class="clear"></div>
</div>
</body>
</html>
See it in action here: http://jsfiddle.net/FVLMX/