non ho ben capito

prova con questo:

<style type="text/css" >
p { padding:50px; }

#sinistra { float:left; background: #000; text-align:center; width:20%; color:#CCC;}
#destra { float:left; background: #333; text-align:center; width:80%; color:#CCC;}

</style>
</head>
<body>
<div id="sinistra">

Sinistra</p></div>
<div id="destra">

Destra</p></div>
</body>