You need a 4 way buffered IDE interface and the IDE Fix 97 software. The interface plugs into the IDE slot on your motherboard and allows you to connect up to 4 IDE devices to your Amiga.... such as hard drives, CD-ROMs, zip drives etc.
The CD-ROM drive would have to be external. External CD-ROM drives are more expensive than internal equivalents but should be available from many Amiga stores such as Power Computing or Eyetech.
The Interface will set you back about £20 (I think thats what I paid for mine a year or two back). An external CD-ROM drive can cost anything from £10 depending on what speed. I'd recommend at least a quad (4x) drive but then again new 52x drives are cheap as chips these days. Its up to you.
