Vertical green stripes when iMac starts to boot up
Vertical bars/strips are usually indicative of a display malfunction. If you are also having booting problems, it is more likely GPU failure. You should take the iMac to Apple soon as it likely requires servicing.

To backup your iMac, connect an empty hard drive with enough storage to back up your iMac, then go to System Preferences > Time Machine. Turn Time Machine on, and select the disk you just connected. Allow your Mac to backup before taking it to Apple.