how to know "which hook" ?

[zodehala]

i try to create new plug-in to fetch (show in postpit) thread count which is posted by X

posbit_legacy :

$vbphrase[total_posts]: <strong>$userinfo[posts]</strong> | Total threads: <strong>$threadcount[count] (<phrase 1="$postsperday">$vbphrase[x_posts_per_day]</phrase>)

hook : showthread_postbit_create

PHP Code:

$threadcount=$db->query_first("SELECT COUNT(*) AS count FROM " . TABLE_PREFIX . "thread WHERE postuserid = '$userinfo[userid]'"); 


but do nothing , and following error

Fatal error: Using $this when not in object context in /forum/showthread.php(1044) : eval()'d code on line 2

how can i know (find out) which hook will i use ? any read me ? any referance ?