Help with general setup and menues

I am having trouble with understanding the general flow of FreePBX and Asterisk - like when a call comes in, exactly where it goes, and which settings (i.e.routes, trunks, etc.) do which things.
Is there any good place to read up on those types of things?
Also, I would like to set up a menu, so when a caller calls, they can hear a greeting, and a list of options, which in turn can route to another menu, or to extensions.
Where can I find and set up those options?
I am using FreePBX with PBX in a Flash.

This should start you off. The PBX in Flash Without Tears is excellent.

The IVR option is where you setup phone trees. You then point your inbound route to the IVR.