Hi everyone - for fish only or fowlr tanks, regular flourescent bulb/hoods should work fine. I would recommend a good protein skimmer too. Dissolved organics could accumulate quickly without one on a 20 gallon.
Assuming you're driving it home from the pet store, Live rock that's been in the dealers tank for several weeks may not need to be cured. You'd have to ask them how long they've had it. If it's coming via air shipment, there may be some curing time involved. Only way to know is with the water test kit Miss Mouse recommended.
I don't personally use base rock since its not as porous as the good stuff and won't harbor as much of the beneficial bacteria. You usually see base rock in much larger setups to save on costs. But using base rock in a smaller tank would really take up much needed space. For a 20 gallon, I'd go with an inexpensive power filter that is easy to maintain, since you'll need to clean out the filter pads on a regular basis. Although canister filters do an excellent job at mechanical filtration I don't think I'd use one on a marine tank (IMO, I'm sure others would prolly disagree).
Also, if you run a good protein skimmer on your tank and have good water flow within the tank, a mechanical filter is not an absolute necessity. The bacteria and other organisms in the live rock and live sand will break down fish wastes and the skimmer will be pulling alot of gunk out of the water column. Some reefers will only periodically run a mechanical filter in order to use activated carbon for polishing the water.
For a list of equipment, see
Saltwater Aquarium Setup for ideas on items needed.
Small tanks require more diligent monitoring, but they can be fun too. Good luck with your tank.

Mike