The only clownfish you can keep in that size tank is Amphiprion ocellaris (common clownfish). All the other species get too big and even the ocellaris will probably be a bit cramped in that tank when mature. However it can be done as long as you monitor the water quality on a regular basis.
Clowns are fine with or without lights on their tank. You can have the lights on for a couple of hours in the evening so you can watch them when you get home from work.
Set the tank up with some shell or beach sand on the bottom. Add some rocks for decoration and fill the tank up with saltwater. You can use natural seawater from the beach. Just make sure it si clean and not polluted. Or you can use artificial salt mix and make the water up that way. You use a hydrometer or refractometer to measure the salinity (salt level). If you use artificial salt mix then it should be made up in a separate container and aerated for 24 hours before use. This allows all the minerals to completely dissolve.
Add something to start the filtration cycle (liquid ammonia or prawn or fish). Allow the tank to cycle (it will take about 1 month). Check out cycling threads. The procedure is the same for fresh or salt water.
Once the tank has finished cycling stop adding the ammonia, do a big water change, and add some clownfish.
If you use live rock (rock from the ocean or out of an established marine aquarium) and live sand, the cycling process will take less time.
