Install Guest Additions

1. Run your virtual machine.
2. Run Devices->Insert Guest Additions CD image....
3. Following the instructions to install Guest Additions.

Setup shared folder

1. From the Oracle VM VirtualBox Manager, select your virtual machine.
2. From the Shared folders section, add you share folder information.

For my case, I shared C:\xuan\vbox\shared folder with the guest virtual machine. In the guest virtual machine, that folder shall be called shared.

Note: If your guest virtual machine is running MS Windows, don't put anything in the Mount point. Otherwise, it will crash your virtual machine.