If it's a keyboard that is less than say 8 - 10 years old it is almost certainly of the membrane switch type. Taking them apart and using only some of the keys is very very difficult one of the reasons being that thee is nothing to solder wires to.
If it's an old keyboard with "Cherry type" real switches, the kind that has a large stroke, and a loud click (they were so much better that the cheap crap you get now 
), you could just hook up each switch to an Arduino digital pin like any other switch.