Setting Cookies in a custom Wordpress Theme

I have developed a custom Wordpress theme, and I would like to add a Cookie that tracks first time visitors to the blog landing page and if they are a firs time blog visitor a modal window would come up giving users some information about the blog.

I found a snippet of code on StackOverflow that tracks first time visitors, and re-directs them to a different page (I would change this, of course)

<?php
if ($_COOKIE['iwashere'] != "yes") { 

  setcookie("iwashere", "yes", time()+315360000);  
  header("Location: http://example.com/index-first-time-visitor.php"); 

}
?>

Now how do I go about implementing my idea? Should I include the cookie script in functions.php and use an action to load the JS required for the modal only when the user is visiting for the first time? Suggest something different? Thanks!