After installing Windows 8 MacBook Air is not booting. I want to revert back to OS X. I need help on how to do that.
press the Option key + R select the disk that installed Mac OSx. maybe you're starting with Windows.
Now if you want to a new installation, you will need access to another Mac, to mount the flash drive installation. follow these steps: http://osxdaily.com/2015/09/30/create-os-x-el-capitan-boot-install-drive/