When you set up a SW aquarium for the first time, there is little to nothing present to eat up the ammonia that fish and inverts create, which will mean any fish you put in there will often die (unless of course you want to keep Mantis Shrimp or Glass Anemones, thier near impossible to kill, but will also kill anything else in the aquarium).
When the shimp decay it releases ammonia, which the Aerobic bacteria feeds on to make Nitrates, which is only toxic in high amounts (some inverts like some corals though will not do well with Nitrates)
Any type of rock will work, doesnt have to be live rock, as long as its porous rock, like Lava rock and Aragonite rock (this is the main part of the biological filter, where the bacteria colonize that break down ammonia, and will also help break down nitrates, unless you go with a biowheel, but that would limit the types of inverts you can get like Coral and Anemones since Bio-Wheels break down ammonia and nitrite into Nitrate at a faster rate then the nitrates can be broken down, if you use a skimmer dont use a bio-wheel)