<div id="entete">Entete</div>
<div id="gauche">Partie gauche</div>
<div id="droite">Partie droite</div>

#entete{
  background-color: silver;
  height: 50px;
}
#gauche{
  background-color: green;
  float: left;
  width: 50%;
  height: 50px;
}
#droite{
  background-color: orange;
  float: left;
  width: 50%;
  height: 50px;
}