Closing up a door and then tiling one side

Hi,

Bit of a novice here, though a confident one! I'm going to knock down the wall between our bathroom and toilet and close one of the doors up. I then need to tile on the inside of the new wall for a shower cubicle.

What I'm wondering is should I use plasterboard or plywood for the inside of the new wall to make it easiest to tile onto and what prep do I need to do to the surface if any?

Thanks in advance.

I've just found some info on aqua board I think it was called, so please ignore my post, this looks like it's what I need.

Thanks.