Virtualbox: How to mount shared folder

create shared folder on host:
Settings / Shared Folders / Adds a new shared folder definition. /
  Folder Path: C:/shared
  Folder Name: shared
  mark: "Auto-mount" and "Make Permanent"
 

Manually mount shared folder:
mount shared folder on guest os (linux):

# mkir /mnt/shared
# usermod -G vboxsf -a oracle
# mount.vboxsf shared /mnt/shared


Auto-mount shared folder on start-up:
# mkir /mnt/shared
/etc/fstab:
shared   /mnt/shared   vboxsf   defaults  0   0