Try
<style type="text/css">
.header
{
width:100%;
background-color:Teal;
}
.left
{
width:50%;
background-color:Aqua;
float:left;
}
.right
{
width:50%;
background-color:Silver;
float:left;
}
</style>
<div class="header">
Head section
</div>
<div class="left">
Left
</div>
<div class="right">
Right
</div>