You could just buy fresh bulbs that are appropriate for saltwater tanks. One 10000K daylight and one blue would be the normal combination. Even if it's only a normal T8 fixture it'll be fine for live rock and even a few very low light corals like polyps and mushrooms. I've even grown toadstool leathers under 2 T8 lights. Once you get the hang of it and have the basic equipment to maintain a saltwater tank you can upgrade the lights if you want to.
I'd really recommend getting a Reverse Osmosis or RO
filter as the most important first purchase for a successful saltwater tank. That is unless you have a store close buy where you can buy good RO
water. They make keeping a tank way easier but for a 20 gallon you can easily just but water. Just don't use tap. A refractometer is great to have for testing the salt levels but a glass hydrometer is passable.