<style type="text/css">
#wrap {
width:600px;
margin:0 auto;
}
#left_col {
float:left;
width:300px;
}
#right_col {
float:right;
width:300px;
}
</style>
<div id="wrap">
<div id="left_col">
...
</div>
<div id="right_col">
...
</div>
</div>
Make sure that the sum of the colum-widths equals the wrap width. Alternatively you can use percentage values for the width as well.
For more info on basic layout techniques using CSS have a look at this tutorial