A list from me to you.
Secure the following
All $_GET and $_POST variables
If its an integer abs(@intval
if it's a string mysql_real_escape_string or $db->escape (use MRES)
Top Priorities
$IP above all else
1. Cmarket.
2. Forums.
3. Userlist
4. Preferances.
5. Player Report
6. Bug Report ( if you have )
7. Viewuser
8. Player Ads ( if you have )
9. Item Shops / itemuse/itembuy.php
10. Use sprintf to clean and trim all data.
11. Secure and Re-name all staff_.php files, stick em in a folder and lock em up.
12. Secure your crons to keep people from running them twice.
If you are unwilling or unable to do all of the above and then some, Delete public_html and get a new hobby or hire a professional, that's what we're for

Hope that helps.
cheers