Steve brings up a controversial point. Live Sand.
The claim is that it contains all the bacterial colonies you'll need, along with other micro organisms. The rebuttal is that after being collected, bagged, stored, shipped, stored again, shipped again, then displayed at the LFS
, there is nothing left alive in it.
Frankly, there is no way to tell if any given bag of live sand contains any life. At best, it contains some bacteria, and some dead matter to serve as an ammonia source. At worst, it's expensive sand.
One way around this issue is to buy dry aragonite sand and layer to your desired depth, then take a cup of sand from an established tank and gently layer that on top. That will contain all the life you need to get started.
The same principle can be applied to live rock. No doubt using all fully cured live rock will speed up the cycle, but at the price it goes for, it can be very daunting. Dry rock is often 1/3 the price. So what many do is purchase MOSTLY dry base rock, along with a few pieces of fully cured live rock. The bacteria and encrusting organisms will very quickly colonize the dry rock and make it "live rock". Granted, this does take a little more time. But, if you add an ammonia source and a bacterial additive, this will be sped up considerably.
By all means get as much Live Rock as you can, as it contains far more than just bacteria. But don't be afraid of using dry rock and sand as well. They become live very quickly. Before long, you'll see asterina stars, micro brittle stars, bristle worms, stomatella snails, and much more!