Lighting is not just provided to enable the fish to be seen, it also acts as a stimulant for the fishes, and is needed by the aquatic plants to photosynthesise. The latter function is very important, because while the plants are photosynthesising they absorb carbon dioxide from the water.
The amount of light is fairly critical, and the final balance between the brightness and the illumination time is best found through trial and error. Naturally most people want to view their aquarium, usually in the evenings, and the plants need a certain mount of light to be able to flourish properly. Usually leaving the light on for 10-15 hours per day will suit both requirements.
Fluorescent lighting is the most common, and these can be bought in a variety of sizes, depending on the length of the aquarium. They normally will be fitted under the aquarium hood, so will need to be shorter than the hood to be able to fit.
The lamps also come in a variety of colour spectrums, some which are brighter and harsher than others, so consider what effect you want in the aquarium. It will depend on the fish in the aquarium and the amount of aquatic plants.