<form method="get" action="<?php bloginfo('url'); ?>"> 
                      <input type="text" value="Arama Yapın .." onfocus="if(this.value==this.defaultValue)this.value='';" onblur="if(this.value=='')this.value=this.defaultValue;"  value="" name="s" id="s"  />
                   <input type="hidden" />

               </form>