Make sure you have installed the guest additions on the guest system.

Definition of the shared folder

Go to the Oracle VM VirtualBox Manager, right-click on the machine and choose Settings.
Create a shared folder in the wizard. Remember the "name" of the folder, in this example SHAREDFOLDERNAME.

If you check "Automount on boot" when defining the shared folder, simply reboot the system and you will find it here: /media/sf_SHAREDFOLDERNAME

Or mount the folder manually

root@testkraxn / # cd /media
root@testkraxn /media # mkdir sharedfolder
root@testkraxn /media # mount -t vboxsf SHAREDFOLDERNAME sharedfolder
Note, I stumbled over this: You need to close the "Add shared folder" wizard in the VBox Manager gui to be able to mount the folder.

Access shared folder with other users

You may want to allow some users to access this dir, so add them to the vboxsf group:

root@testkraxn /media/cdrom # usermod -a -G vboxsf tester
root@testkraxn /media # groups tester
tester : tester adm cdrom sudo dip plugdev lpadmin sambashare vboxsf