<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<div><input type="text" size="18" value="<?php echo wp_specialchars($s, 1); ?>" name="s" id="s" />
<input type="submit" id="searchsubmit" value="Search" class="btn" />
</div>
</form>
If your templates supports it you can just add:
<?php include (TEMPLATEPATH . '/searchform.php'); ?>
(like "Write your search and hit Enter" )
<form method="get" id="searchform" action="<?php bloginfo('home'); ?>/">
<div><input type="text" size="put_a_size_here" name="s" id="s" value="Write your search and hit Enter" onfocus="if(this.value==this.defaultValue)this.value='';" onblur="if(this.value=='')this.value=this.defaultValue;"/>
<input type="submit" id="searchsubmit" value="Search" class="btn" />
</div>
</form>
