If I explained everything you should know about saltwater to get started, I'd have a thick book. But, here are some things to do a search on. And then if you have more specific questions, we can narrow it down. Also, most suggest the consientious marine aquarist, or i know there's a dummy book out too, saltwater fish for dummies. It explains things in common terms. Basically read as much as you can. I've been into saltwater for about 4 months now and feel as if I know next to nothing.
Filters
quarantine tanks
care for anenomes
skimmers
nitrogen cycle
test kits
aerating water/aged water
live rock and base rock
live sand and argonite sand
crushed coral
DSB (deep sand bed)
clean up crew.
As for your more specific questions, you'll only be able to keep a limited amount of small fish in your aquarium. At first the ratio is one inch of adult fish per 8 gallons, for at least about 3-4 months, then about one inch of adult fish per 4 gallons, and after about a year and you feel comfortable, about one inch of fish per 2 gallons. I would wait at least a year before going into corals, anenomes, and things of that sort, and about 6 months before starfish. This is because all of these things require near perfect water conditions.
