First it would depend on the voltage of the system- 6 or 12v, there might be more. Standard batteries (AA, C, D) are 1.5 volts each. If you have a 6v light you will need 4, 12v would need 8. D cell will last longer than AA.
Once you figure out hov many batteries you will need, time to mount them. They need to be mounted in series (+ to - , + to -). You can get battery holders at Radio Shack or other electronic shops. The light originally used the bikes frame as the ground and you can do the same by connecting the negitive side of the battery to the frame (needs to directly contact the metal of the frame, no paint or it won't work). The positive wire from the batteries will need to run to the switch on your light if it has one. If not, get one while you are at Radio Shack. The other terminal on the switch will run to the bulb (in the switch is mounted inside the light housing) or to the terminal on the light where where the wire from the generator was mounted.
Hope I didn't confuse you too much. It was kinda nice to put my education to use.