This hobby requires a great deal of foresight and planning ahead. You need to decide if you want a tank that only has fish (FO), a tank that has fish and live rock (FOWLR), or a reef tank (has corals). If you are just going to do a FO or FOWLR then you won't need to upgrade your lights. Do you have one or two 48" bulbs? The only reason you would need to upgrade your lights is if you plan on keeping corals, clams, or anenomes. If you just want to start with the basics, you need to get a wet/dry filter, an over the back overflow box, a protein skimmer, a sump is a good idea, but you can get away without one, and add it later. you will need to switch your substrate if you used the tank for FW. you should get argonite sand for your substrate and get rid of the gravel you probably have now. You will also need a hydrometer to measure your salt level. Let's see, loose the undergravel filter. You'll need the following test kits which are often sold together: ammonia, nitrite, nitrate, high range pH (different from FW pH test). There are many many more things you will want in the near future, but if you have these, you should be ok to start out with. You should also strongly consider getting an RO filter for your tap water.
