How to set a cookie in Wordpress

I'm trying to set a cookie in wordpress. I have my cookie set like this :

<?php setcookie('test', 'test', 0, '/', '/');  ?>

in header.php of my theme, but when I go to my browser to view my website I get this error

Warning: Cannot modify header information - headers already sent by (output started at /home/content/19/9468119/html/wp-content/themes/twentyeleven/header.php:27) in /home/content/19/9468119/html/wp-content/themes/twentyeleven/header.php on line 201

and also my cookie doesnt set. How do I set a cookie in wordpress?

I have also tried this

 function set_new_cookie() {
    setcookie('test', 'test', 0, '/', '/');
}
add_action( 'init', 'set_new_cookie');