Shooting in the dark abit here because you have not provided much info regarding your system, and the reasons for the second disk.
If you just want more storage because your first disk is filling up then I would recommend that you stick with IDE, which is what I am guessing your system has in it at the minute. Its fairly easy to add a second IDE drive, just open the case, screw it in and attach it to your IDE cable. (Your cable may also need replacing if it does not have a spare port on it, they are dirt cheap to get)
Adding a SCSI would be a lot more difficult, as you would have to get a SCSI card first of all to attach the drive to. Also SCSI drives are a lot more expensive than IDE drives, and for normal everyday useage the performance boost is not worth the extra cost in my opinion.