Brand new and more than a little confused.

[Aquattro]

This is the fun part for sure. For one, I wouldn't personally stuff 75 pounds of rock in a 50g, I'd go max 50 pounds or your fish won't have anywhere to swim. Also, the cannister should go, sell it and put the money towards something useful (they can contribute to high nitrates).
Buying used rock can be fine, but I would watch for old rock (over 3 or 4 years) and rock that looks freshly scrubbed (someone may have just scrubbed off lots of hair algae to sell it). I'd buy a box of fiji rock from one of our vendors here, it's about 50 pounds at less than 300 bucks. I always go uncured, but I'm a sucker for punishment. I like to give the remaining life the best chance it has, which I think I can do better than the LFS.

Go slow, and everything will fall into place.