I've tried searching and figuring this out, but I'm getting mixed answers so:

We're planning on flying NYC-MEL middle/late Feb with 3 month old baby. We will be flying Business (thankfully). Probably United, but Qantas based answers are helpful.

Do we need to buy an extra seat for the baby?
If so, is it 10% of full price?
If not, do we just put the baby in a carrier (car seat?) on the floor. Do we hold him during TO/Landing?

Thanks.