For garbage disposal wiring, it is best to first check the electric code on a disposal circuit at your city building department. They may advise you as to the specifications for this job, but at a cost. It will probably require a permit. The good side of the permit is a professional will inspect the job and assure your safety.
In our area, the disposal must be plugged in to a switched receptacle. This way the disposal with a pigtail, a short extension cord, can be unplugged if the switch ever malfunctions and the unit is stuck on. Alternatively, for extra safety the unit can be unplugged before dislodging any debris jamming the grinders.
If there is no switch above the counter, you will have to add one. Using an old workbox, you can install one from the surface without making an oversized hole. Another hole made in the cabinet below and you should be able to fish Romex wire from below up to the switch box.
If the new power comes from below, a switch leg will control the receptacle. This happens when power first goes to the receptacle below the counter, then up to the switch. Turning the switch on completes the circuit of the black wire back to the receptacle.
Consult an expert for exact details for your particular wiring situation. This ranges from home repair books to personal guidance from professionals. With the proper planning, this can be an easy job.
Attach a grounded 16-gauge pigtail to the disposal before installing it to the sink. Use a strain-relief where the cord goes in to the base of the disposal. Connect black-to-black wires, white-to-white wires with orange wire nuts, and connect the ground wire to the metal ground screw.
Make sure the switch is off, install the disposal and plug it in. If all is well the disposal should turn on and off with the switch.