I don't mean to sound harsh but if you want to know if turtles need the light, I'd suggest a LOT more research. I mean this is a very long term commitment, and after the large tank, filter, lights, heater, UV tube etc you're looking at a rather substantial financial outlay, not to mention the responsibility that comes along with holding a life in your hands, and being accountable for it.
I can only hope the store person will let you know how much work is actually involved with keeping any turtle, and provide sufficient advice with regards to the equipment you will need.
It is also important to research common ailments and preventative measures, like the need for calcium.