What file to edit Header?

I have recently added a forum to my site and want to include a link on the top of the Zencart header. Currently there is "Home" and "Login" at the top. I simply want to add "Forum" along side these links at the top. How can I add this: what file and where is it located? Thanks in advance?

Actually once logged in there are several other links that appear on that line
I would make your forum link last

In a copy for your template structure (override) of tpl_header.php

Thanks. I have found the file. What is the appropiate method to insert my forum link at the end without causing errors? Sample code would be appreciated.

<li><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGOFF; ?></a></li>

<li><a href="<?php echo zen_href_link(FILENAME_ACCOUNT, '', 'SSL'); ?>"><?php echo HEADER_TITLE_MY_ACCOUNT; ?></a></li>

Try this, using a code editor insert the red highlighted code correcting for your details

<div id="navMain">
    <ul class="back">
    <li><?php echo '<a href="' . HTTP_SERVER . DIR_WS_CATALOG . '">'; ?><?php echo HEADER_TITLE_CATALOG; ?></a></li>
<?php if ($_SESSION['customer_id']) { ?>
    <li><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGOFF; ?></a></li>
    <li><a href="<?php echo zen_href_link(FILENAME_ACCOUNT, '', 'SSL'); ?>"><?php echo HEADER_TITLE_MY_ACCOUNT; ?></a></li>
<?php
      } else {
        if (STORE_STATUS == '0') {
?>
    <li><a href="<?php echo zen_href_link(FILENAME_LOGIN, '', 'SSL'); ?>"><?php echo HEADER_TITLE_LOGIN; ?></a></li>
<?php } } ?>

<?php if ($_SESSION['cart']->count_contents() != 0) { ?>
    <li><a href="<?php echo zen_href_link(FILENAME_SHOPPING_CART, '', 'NONSSL'); ?>"><?php echo HEADER_TITLE_CART_CONTENTS; ?></a></li>
    <li><a href="<?php echo zen_href_link(FILENAME_CHECKOUT_SHIPPING, '', 'SSL'); ?>"><?php echo HEADER_TITLE_CHECKOUT; ?></a></li>
<?php }?>
    li><a href="http://your_domain.com/your_forum" rel="nofollow">My Forum</a></li>
</ul>
</div>

Thanks for the help, that worked. One more question for you please. What file should I post my Google Analytics code into? /includes/languages/english/index.php? Thanks agaiin

Can be added to tpl_footer.php