http://jsfiddle.net/kkobold/qMQL5/
#header {_x000D_
width: 100%;_x000D_
background-color: red;_x000D_
height: 30px;_x000D_
}_x000D_
_x000D_
#container {_x000D_
width: 300px;_x000D_
background-color: #ffcc33;_x000D_
margin: auto;_x000D_
}_x000D_
#first {_x000D_
width: 100px;_x000D_
float: left;_x000D_
height: 300px;_x000D_
background-color: blue;_x000D_
}_x000D_
#second {_x000D_
width: 200px;_x000D_
float: left;_x000D_
height: 300px;_x000D_
background-color: green;_x000D_
}_x000D_
#clear {_x000D_
clear: both;_x000D_
}
_x000D_
<div id="header"></div>_x000D_
<div id="container">_x000D_
<div id="first"></div>_x000D_
<div id="second"></div>_x000D_
<div id="clear"></div>_x000D_
</div>
_x000D_