HI,
So this is a different site and I don't think a rel canonical in the index.php is what you want, especially for a www/non www duplicate issue which is best solved with an htaccess 301 redirect. I am not familiar with that forum software but I would be very careful about putting canonical tags in if you are unsure what a particular file is or where it will be displayed - you might end up with the same tag for the entire site which would not be good at all!
Its best to try to take it one thing at a time, do a correction you are sure will fix one of your duplicate issues and then maybe wait for the next moz crawl which will have less issues and make it easier to focus on the remaining ones and investigate on the best way to solve them.
For the www or non www redirects you would usually put a couple of lines in your .htaccess file in your root directory, you can check out this link for the exact code and how to do it through cpanel. I would suggest taking a backup of the existing file in case you need to restore it quickly (typos can take a site down easily). If you have a developer who is familiar with this kind of thing it might be worth asking them to assist.