How to install windows 8 on my lenovo g500 laptop?

According to the Lenovo site http://px1.us?3LU, your laptop doesn't have a DVD drive. You'll need to get an external DVD drive.

If you have not DVD room you can set up your windows with your pen drive at least 4gb or higher. Just make Booth able USB pen drive… First go to enter boot option then firth boot select removable disk or USB then restart pc before connect USB

Are you using a Windows 8 boxed install disc set?
Insert W8 disc one into the CD/DVD drive log off and turn off the laptop then restart it and press a key. Have it plugged in using the power adapter do not start install on battery power, do not stop the install once you begin. Do not remove or change the operating system recovery partition as it can't be fixed or recovery partition reinstalled.

How to perform a clean installation of Windows http://windows.microsoft.com/...an-install

Install, upgrade, & activate Windows 8 http://windows.microsoft.com/...nvistatab1