The first factor that determines the necessity of aquarium lighting is whether you have real plants in your tank or not. If yours is a planted tank, the aquatic plants in the water will require light to perform photosynthesis. Without proper lighting at least for a couple of hours of each day, your live plants will begin to wither and die. If your aquarium is positioned in a place that receives some sunlight daily, even if it is not direct sunlight, then fish tank lights aren't really that crucial.
Even if you do not have live aquatic plants, having a tank that sits in a dark corner of an ill lighted room is not healthy to your fish. Fish like all living creatures live in a cycle of light and dark. Without a distinct difference between the two, they would eventually have deteriorating health and other issues.
In general there are three types of aquarium lights that you could get:
Normal Fluorescent Lights:These are the common types of lighting you can get for a very cheap price in any pet shop. The normal fluorescent lights do provide the basic function of lighting up your fish tank and they do help with providing your aquatic plants the light it needs. The issue about these lights though is that they can be a little bulky. If you do get shorter tubes, the strength of the light might not be sufficient. The same goes if you have a fairly large tank, where you will need to get a couple of tubes together in order to provide suitable illumination.
Compact Fluorescent Lights:These are just like the normal fluorescent ones with the exception that they are much smaller. But don't let the size full you, they do provide sufficient illumination as much as a longer normal one would. So the benefit here is that you would be able to have more tubes in a smaller space, giving your plants and fish all the light they need.
Metal Halide Aquarium Lighting:These are the latest addition to the aquarium lighting scene. Metal halide lights are able to provide illumination that is stronger and more concentrated. This allows the light to penetrate far deeper into the tank than Fluorescent lights can, making metal halides perfect for really large tanks. The light spectrum that they provide is also ideal for plants and can be selected based on the type of aquatic plants that you have.
The only drawback here is that it is for advanced aquarium enthusiast because it needs proper installation and maintenance. So if you are a new hobbyist and do not want to go through the hassle, this may not be the one for you.
So your task now is to evaluate the size of your fish tank and see which of the three options you would like to go with. They all have their pros and cons, so it is mainly a matter of preference.