How can I add a jQuery OnClick event to the Publish posts button?

You can hook into the post footer actions (based on this answer, not tested):

add_action( 'admin_footer-post-new.php', 'wpse_80215_script' );
add_action( 'admin_footer-post.php', 'wpse_80215_script' );

function wpse_80215_script()
{
    if ( 'post' !== $GLOBALS['post_type'] )
        return;

    ?>
<script>
document.getElementById("publish").onclick = function() {
    if ( confirm( "Ready?" ) )
        return true;

        return false;

}</script>
    <?php
}

These actions are called in wp-admin/admin-footer.php:

do_action( "admin_footer-" . $GLOBALS['hook_suffix'] );

This code can be used in a plugin (preferred) or in your theme’s functions.php.
See also:

• Where to put my code: plugin or functions.php?
• Where do I put the code snippets I found here or somewhere else on the web?