I wan to lay down a new ceramic tile floor over 3/4 plywood. What shoud we use as an underlayment?

If this is t-g plywood both glued and nailed or screwed to the joists, you are at the minimum for ceramic using your choice of any 1/4" cement board underlayment set into a thinset bed and screwed down or any sheet membrane. If it's square edged plywood, you will need to either add solid blocking between the joists to support the panel edges where they span joist bays or add another layer of 3/8" because plywood prior to cement board or membrane.

Use 1/2'' cement board. Overlap long seam of plywood. This will eliminate the blocking .
Rest of installation same as Bljack stated in post#2

Chuck