This post explains how to install Guest Additions on a Lubuntu Linux OS installed using Oracle VM Virtual Box. If you want to know how to install Linux image using Virtual Box please refer below link.
Guest additions adds extra features to guest OS like clip board sharing, folder sharing etc. To enable guest additions for the user make below changes.
Select Devices -> Insert Guest Additions CD Image from the Virtual Box Menu Bar
Click on OK
Execute below commands in Terminal
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install gcc make perl
Open Users and Groups and click on Manage Groups
Select vboxsf from the groups and add <user>as group member
Enable guest additions options from Devices -> Shared Folder, Devices -> Shared Clipboard
Reboot the guest os to make changes effectively