hi ill be changing server soon i just need to know if when ive installed mybb do i need to install all the plugin`s again or do they go on with the back up?
You do not have to reinstall the plugins. Just move the website to the new server by importing the website's backup.
but i still need to install mybb on the new server dont i?
simply download all your files from the current server. upload to the new server. export your database from your old host using phpmyadmin create a database on your new server then import the backup there. you will probably need to amend the database settings in the /inc/config.php file to reflect your new database settings
also cookie settings in your /inc/settings.php file should reflect your new server
$settings['cookiedomain'] = "";
$settings['cookiepath'] = "";
$settings['cookieprefix'] = "";
that should get it running for you.
i am pretty sure thats all you need to do. definitely worth trying this on localhost first though in my opinion