"Right so even if delete an assignment, the files that were uploaded will still be on the moodledata on the recycle bin because moodle creates a backup for everthing that was deleted right?"
Correct.
"so i need to run cron to get them deleted right?"
With changes to recyclebin made first ... yes.
"is there gonna be a problem to run cron now? i need to put moodle on maintenance to do it?"
So you've never setup cron? Running it now will probably result in server being slow but what choice do you have? Can't run cron while site is in the maintenance mode.
Do you have ssh access to server? Cron jobs have a lot to catch up upon. Issue a warning to users/admins that the site will be slow ... and there is nothing you can do about that!
cd /path/to/code/admin/cli/
then to be sure it runs til completed:
nohup php cron.php &
That will put php running cron in a no hangup mode ... output of the that will go to an .out file at that location. It will run even if your ssh session ends.
After it's completed, setup cron to run once every minute.
https://docs.moodle.org/310/en/Cron
'SoS', Ken