I suck at MySQL JOINS, can someone gimme a hand?

[Cloudrunner]

How would I compress these four queries into one?

PHP Code:

$nav_new_users_today = $vbulletin->db->query_first("SELECT
    COUNT(*) AS count
    FROM ".TABLE_PREFIX."user
    WHERE joindate >= '".$nav_starttime."'");
$nav_users = $vbulletin->db->query_first("SELECT
    COUNT(*) AS count
    FROM ".TABLE_PREFIX."user
    WHERE lastactivity >= '".$nav_starttime."'");
$nav_new_threads = $vbulletin->db->query_first("SELECT
    COUNT(*) AS count
    FROM ".TABLE_PREFIX."thread
    WHERE dateline >= '".$vbulletin->userinfo['lastvisit']."'");
$nav_new_posts = $vbulletin->db->query_first("SELECT
    COUNT(*) AS count
    FROM ".TABLE_PREFIX."post
    WHERE dateline >= '".$vbulletin->userinfo['lastvisit']."'"); 


I came up with a LEFT JOIN scenario, but alas, due to my lack of knowledge on this, it failed...

PHP Code:

$nav_query = $vbulletin->db->query_first("SELECT 
    COUNT(new_user.*) AS new_users_today, 
    COUNT(thread.*) AS new_threads, 
    COUNT(post.*) AS new_posts, 
    COUNT(user.*) AS users 
    FROM ".TABLE_PREFIX."user AS new_user 
    LEFT JOIN ".TABLE_PREFIX."thread AS thread ON(thread.dateline >= '".$vbulletin->userinfo['lastvisit']."') 
    LEFT JOIN ".TABLE_PREFIX."post AS post ON(post.dateline >= '".$vbulletin->userinfo['lastvisit']."') 
    LEFT JOIN ".TABLE_PREFIX."user AS user ON(user.lastactivity >= '".$nav_starttime."') 
    WHERE new_user.joindate >= '".$nav_starttime."'"); 


Any and all help on this would be greatly appreciated...I'm just straight up stuck, I've been through all of my reference materials, but just cannot figure out how to do this...

Thank you in advance for the assistance!

)O( Cloudrunner )O(