Substrate: Some use crushed coral, other's use a (DSB) Deep Sand Bed and others go barebottom. I would suggest that you go to ReefCentral and do some searching on those i've mentioned. There is a big debate going on about DSB's right now. You'll find a ton of info.
Skimmer: A skimmer, as I understand it, will take nasties out of the water before they can turn into Ammonia, Nitrite, etc. In any case, it's a good idea. I have a CPR Bak-Pak skimmer that we use, it does a good job. Alot of people rave about AquaC Remora skimmers. Either would work on that tank size.
Live Rock: You should should for 1 to 1 1/2 lbs. per gallon. Many different kinds of rock.
Lighting: If your going to do a fish only tank, lighting isn't really that important. If your doing a reef, with corals, lighting is VERY important. The lighting you get will depend on the corals you want to keep. Different lighting are (PC) Power Compacts, (VHO) Very High Output, (MH) Metal Halides. Also do some searchs on lighting and you'll find a ton of info.
Salt: I use Instant Ocean salt, never had a problem with it. Some debate over salt mixes going on also.
My suggestion is research, research and research some more. Paticence is best if Saltwater Aquaria. Nothing good happens fast. I hope no one gets mad at me for referring you to
www.reefcentral.com , but they are a big site with LOTS of good info. Personally, I like to go to as many boards as possibe, variety is good. Good luck and keep us posted. This should be enough to keep your head spinnin for awhile.
