"1) What is the customs/immigration procedure?"
Same as any US airport. 1st , go through immigration, then collect baggage, then clear customs. If you arrive on either flight QF155 or (i believe) the new non stop service (QF175) from BNE, then you will arrive and clear customs/immigration in the Tom Bradley International Terminal. All other flights clear customs/immigration in T4
"2) Is there a seperate line for 1st Class ?"
Nope. If your flight arrives at T4 then there is a separate carousel for the luggage for 1st/Business/Priority
Dave