Best advice....see our friends on the tile forum, johnbridge.com
From my personal experience, there are couple of products you might use:
A few companies make membranes. They are intended and designed for direct application of the tiles. There is also a paint-on coating called RedGard which also works well.
Most importantly, tile needs to be on a very rigid substrate. Any flex will cause tile cracks. Generally, a 1" minimum thickness of subfloor is recommended.