How do i Redirects users back to my forum?

[Yellow Slider]

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.

[Dismounted]

You can't fit to Google's demands is you redirect back to the main page. Google wants a 404 status, but because the main page "exists", it gives back a 200 (OK) status.

[Eikinskjaldi]

What dismounted said. Basically, dont mess with 404.

As for the other question, put this in your .htaccess

RewriteEngine on
RewriteCond %{HTTP_HOST}   !^www\.mysite\.com [NC]
RewriteCond %{HTTP_HOST}   !^$
RewriteRule ^/(.*)         http://www.mysite.com/$1 [L,R]

[Yellow Slider]

how can i redirect back to the forum if the page doesn't exists?

and thanks.

[Dismounted]

You said you want to adhere to Google. You can't do that if you redirect to the front page.

[Yellow Slider]

Your are basically saying that Google won't allow to redirect back to the main page if a page doesn't exists ?

[Dismounted]

Yes, because it gives back a 200 status. Google wants a 404 (Page Not Found). Otherwise, every page would "exist".