As preparation you need:
1. Lubuntu .iso (download if from lubuntu website)
2. A usb disk (8GB and more) prefereably USB3 for speed
Launch VirtualBox and create a new Virtual Machine, put "Lubuntu" as name then...
...press [Next] until ask for "Hard Disk" and select [Do not add a virtual hard disk] until virtual machine is created
Once created, run your virtual machine; you'll be asked to select a start-up disk. Click the proper icon and select your Lubuntu iso
After the language selection select [Install Lubuntu] and right click the small usb icon and select your USB key
Now, just follow the install instruction (usually just press [Continue] ) until the "Installation type" screen and select [Erase **WHATEVERWHATEVER* and re/install]
complete the installation, reboot PC from USB (check your bios/mainboard's manual for info), enjoy