How can i make a ceramic piezo operate at a specific frequency?
You need a circuit that runs the piezo at its mechanical resonance, which is most often 1.6-1.75MHz for this application, but can vary from 1MHz to 2.5MHz. Higher frequencies result in smaller droplet size, proportional to the wavelength. Typically at 1.7MHz you'd expect droplets in the 2um range. 

Here is a typical circuit, from this source, where they have been kind enough to give the actual values for a particular implementation: 

 

The circuit includes a shut-off for low water level. 

The entire oscillator is a single-transistor affair consisting of the BU406, the piezo element and some passive components. 

You might also find Patent US3989042 Oscillator-exciting system for ultrasonic liquid nebulizer useful. 

Typical impedance of a 20mm diameter element at resonance is around 50 ohms. The mechanical mode for this application is thickness, rather than the bending mode that most people are more familiar with for audible piezo elements.  

As the patent I referenced indicates, it's not a very good idea to try to drive the element with a fixed frequency as it would have to be 'tuned' to the particular element's resonant frequency in order to get acceptable performance. The self-oscillating circuit automatically oscillates near the optimum frequency set by the characteristics of the element itself.