Substrate question

[michels]

Hello. My name is Michel and I am a newbie. I am setting up a new marine aquarium and I need help with a few items. Although I intend to add corals to my new tank, my first goal is to run a fish only live rock tank for six months or so, until I feel totally comfortable with the process. My questions deal with live rock and substrate.

I am building a 120g tank. If I recall my readings, the rock should be placed on the bare bottom and then, the substrate added. Here are my questions:

1. Since I intend to add corals, I need to build rock shelves or locations where the corals will be anchored. How do I estimate the amount of live rock that I will need? Is there a rule of thumb?
2. I've already decided that I want something on the bottom of the tank (no bare bottom). Should I invest in live sand or will the live rock, given time, populate the substrate automatically?
3. How do I calculate how much sand I need?

That's it for now. Any advice would be appreciated.
Thanks.