how can i add a forum without going on the acp? is there a plugin?
No, you've to do this from ACP.
Not really sure why you'd want a plugin to basically replicate an entire module of the ACP...
If you really don't want to do it through the ACP, you could do it through PHPMyAdmin. Go to the database your forum is on and run an insert query on the forums table.
If you do that you still need to rebuild the forums cache, which still means going to the ACP. Adding things directly to the database is hardly easier than using a form in the ACP.