How do i Redirects users back to my forum?

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If a user will try to access http://www.mysite.net/notexits
and the file isn't exists, how can i redirect this user back to my site?
i don't want him to see the 404 file, in addition if some one is getting to my site with
http://mysite.net/ and i want him to access with the www, can i redirect that too?
I now that those options are available in vBSEO , but i don't want to use it..
i just need to add some lines in the .hataccess file, but what to put? i want it to fit to Google demands , about redirecting etc'..

Thanks a lot.