I've downloaded the
.iso file from Ubuntu's website.How can I use it to install Ubuntu?
I've searched a lot but every where the instructions are installing Ubuntu from windows.
Note: There's No OS in my computer.
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You need to burn the iso to a blanck CD/DVD or make a bootable USB, and then boot from this removable media to install on your PC.
In general you need another PC to perform this action, or you can demand an Ubuntu DVD from some shops and use it.
In this community wiki you can find different methods to perform that.
To make bootable usb using ubuntu: http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-ubuntu
To make bootable USB using Windows:http://www.ubuntu.com/download/desktop/create-a-usb-stick-on-windows