How do I fill holes around a light switch?
Pick up a small bucket of an all purpose patch likeDap Flexall All Purpose Filler. The all purpose stuff is a little thicker than typical joint compound, so it makes filling a bit easier.
Remove the switch cover.
Using a taping knife, fill in the holes by pushing the compound into the hole and drawing the knife away from the hole. 




Like so...




Repeat for each hole.
Wait for patch to dry.
Sand, Prime, Paint, Reattach the face plate.




If you find the holes are too big, and the compound is just falling through the hole.  You'll have to get a wall repair patch kit, to fill the holes before covering them with compound/plaster.  Follow the directions on the patch kit.