If you are really serious about setting it up as a saltwater tank, ditch the undergravel filter, they are terrible. Go with a hang-on-the-back filter, as well as 15 lbs. of live rock and aragonite for the bottom (so that you can do water changes often to keep it clean since you cannot run a protein skimmer on there). Get crabs, hermits, snails, and a few stars to keep the bottom clean as well. You can put one or two small fish in the tank (a pair of clowns would be nice). For lights, get a nice power compact fixture, preferably one that can offer you at least 40 or 50 watts (which would be 4 ot 5 watts per gallon). Then, you will be good to go and you will be able to keep some corals in that size tank as well (preferably softies like mushrooms, leathers, fingers, polyps, etc.)
Thus, you need a hang-on-the-back filter, a bag of aragonite for the substrate, a power compact fixture that can offer at least 40 watts of light, 15 lbs. of live rock, and a good brand of salt mixture, such as Oceanic. From there, you will be good to go.
Make sure you get uncured rock and after you set the tank up and put water and the correct amount of salt in it, connect your filter, put the aragonite on the bottom, put your 15 lbs. of live rock in there and do not add anything else to the tank for 4-6 weeks to allow the tank to cycle! Following the 4-6 week period, test your water and if your ammonia and nitrites are definitely 0, do your first water change of about 30-40% to lower your nitrates. Make sure your pH is correct too, around 8.3, and your specific gravity is about 1.023. When you have done all this and all the water levels are right, first add your clean-up crew of snails, crabs, hermit crabs, and maybe a small sand-sifter star to sift through the aragonite. Let them in the tank for another couple of weeks, monitoring the tank to make sure everything is holding up well. In about another month, add your first fish (or pair of fish if you decide to go with a pair of clowns).
Just remember, take the process slow and do not rush into anything! Do not add a lot at any given time, because it is such a small tank.