What kulpreetsingh said is absolutly correct.
I just wanted to add two small things for other people’s sake also :
1. if your template supports it you can just add :
<?php include (TEMPLATEPATH . ‘/searchform.php’); ?>
2.adding the following code will put a text inside the search box.
(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>
and finally, combining those two advices, just save the code from part two as searchform.php in your template folder, and you can always call it with method number 1.