You need an acsi-scsi-converter as Anders wrote. It's a piece of hardware that provides the possibility to use scsi for the atari-computers which are fitted with acsi. This converter is connected to the acsi-port of the computer or even the last atari-harddisk, -cd-rom, -printer or whatever.
After connecting it you can plug 'normal' scsi-devices to the atari.
Sometimes those adapters are sold at eBay. I've got several of them to use my scsi-devices on my ataris.
I own an older icd-adapter, one adapter that was fitted into a profile44 (a syquest drive) and another one that I don't know if it is o.k. as I just got it some months ago but am into macs at the moment. :-)
I also had one fitted into a MegaSt with an internal 40 meg harddrive.
And last I've got a converter out of an old sh205. In those harddisks there was an acsi-scsi converter with an additional harddisk-controller for mfm-disks. In the later harddisks atari sold these two were built into one single board.
If I confused you now feel free to contact me via e-mail to shoot a bunch of questions at me ;-)
