You didn't say if you had just a plywood sub-floor, or a plywood floor IN ADDITION TO a sub-floor. It will make a difference. But, no matter ...
Check out this link on Ceramic Tile. Then click the URL for Floor Underlayment. In there, you'll find a discussion about what you need for underlayment when doing a ceramic floor.
If you have more questions, post up. My best to ya and hope this helps.
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