Which cabin are you travelling in? Which flight number (some flights are operated on the 777-200, others on the 747)?
If you are in world traveller on the 777, there are two bassinet positions at seats 26DEF. As for how to get the seats allocated, I'm not 100% I'm afraid.
If your child is 6 months or under, then you will have a bassinet (cot), then 6 months to 3 years you can have a car seat (both available on board.