I am seeing this in a different manner that Wannabbee MAYBE.
If this garbage disposal is no more than a 120 volt 1/2 horse power motor then you will be less than 50 % of a 20 amp branch circuit.
Confirm that the light switch [take this switch to be a light over the kitchen sink] is a 12 awg 20 amp rated branch circuit that has no receptacles that are located in the kitchen, nook, dining, or pantry areas. If that counter light switch is on a lighting circuit or a general use receptacle circuit serving living areas and not any receptacles in that kitchen, nook, dining, or pantry area.
If this is a lighting circuit and you have no kitchen receptacles on that switch then you may be able to use that switch again only if you find two romex cables in that switch box. The NEC would allow it because a 1/2 horse power motor pulls 9.6 amps less than the 50% Wannabbee was thinking.
Bad news is that the inrush garbage disposal motor will cause any incandescent light bulbs to dim when that garbage dsiposal motor is starting up to run. Suspect that though is why Wannabbee suggested running a single circuit.
If all the above meets you situation then you would be meeting minimum safety standards to add the garbage disposal on that switch over the counter if all situations discribed above are as suggested.
Best idea is to do as Wannabbee suggested. However we try to tell you what the minimum safety standards allow then suggest a better way and let you make the decision how much more than minimum you want to go. Do you want to put up with the dimming of lights to avoid fishing a new branch circuit to that garbage disposal. Are you planning on a dishwasher as Wannabbee mentioned. If the answer to that first question is no or the answer to the second question is yes then you should follow Wannabbee's suggested single branch circuit.
If you plan to combine the dishwasher and garbage disposal then you need a noncoincidental load ruling by your AHJ to allow you to connect dishwasher and garbage disposal on same 20 amp branch circuit. I suspect you will find your dishwasher may exceed the 50% load limit for more than one motor on a single circuit unless you use the noncoincidental rule of both motors not running at the same time or the garbage disposal not running long enough to affect that circuit adversly. This would be you local inspector's place to make that ruling before you start.
Hope this helps