You can place the code in the header.php file within the head section,
but after the jquery.js file. However, if you only need this code on
one page, you could just place it in that page within script tags. As
long as you are using $(document).ready(function(), it doesn't matter
where it is placed (head section, body). Although some like to place
their javascript in the head section.
Alternatively you can have a php condition in the header.php file. For
example:
if ($page == '#PAGE_ID') {
// your jquery code
}
