ok mr sakura, now u need to follow the following steps:
1. either copy that file to the pen drive or dvd/cd(burn).
2. restart computer with the booting device usb drive(if u had file in pen drive) and dvd/cd if u have in dvd.

then computer will boot from pen-drive...or dvd whichever u select... and u can install it .....

if u couldn't able to do this.... redownload the ubuntu again... ur file may corrupt....