Calculating straight time and overtime

I'm pretty new at the whole Excel thing and many of the formulas are way over my head.
I'm trying to figure out how to make a time sheet calculate straight time and overtime and I can not figure it out. The time sheet from the Excel Bible was close, but not quite what I'm looking for.

I work seven rotating twelve hour shifts and anything over eight hours and anything over twelve hours is overtime. The shifts start at 0600 and 1800.

I have several questions.
1. how do I get the times to calculate? It works correctly when doing one shift (days), but not the other (night). It adds up to 24 on night shift.
2. how do I make it seperate the twelve hours into 8 straight time hours and four over time hours?
3. how do I get it to know when it has reached the 40 hour mark?