A protein skimmer removes DOCs (dissolved organic compounds) from the water. So basically, it makes the water cleaner =). Depending on the tank and inhabitants you get, you may or may not need one.
I would recommend a fish only tank with live rock (FOWLR). Live rock requires no extra care and provides a natural environment for your fish.
As for the tank and equipment, it's pretty much up to you. People will give you lots of different opinions, but it comes down to what works best for you. I myself will be getting a 20 gallon glass tank, Eclipse 2 hood, live rock, two clownfish, a black clown goby, a cleaner shrimp, a scarlet leg crab or two, and someday a Chocolate Chip Starfish.
I chose a glass tank because acrylic tanks are more expensive and also scratch really easily (I've heard the scratches are a pain to remove, too). The Eclipse 2 wouldn't be good for a reef aquarium because of the lighting, but since I don't plan on having a reef tank, it should do the job just fine.
There is lots and lots of useful information in these Message Boards. If you have questions, I would recommend using the Search function at the top righthand corner of the page.
Here is a website with an equipment checklist that you may find helpful:
http://www.saltwaterhobbyist.com/faq/checklist.htm
Good luck with your aquarium!
-lefty
