I presume that every time the piezo sensor is vibrated back and forth it generates a voltage. As you mentioned, use a PIC or an Arduino micro and take the output from the piezo directly to an input pin of the microprocessor (or take it through a transistor first if the signal is too weak).
That sounds straightforward enough, especially on something like the Arduino Uno or even the smallest Pic.
On the Arduino, you would program a timer to time in milliseconds until you detect the next voltage spike - or perhaps time a few spikes and find the average period (T). To calculate the frequency in Hz, one just takes the
inverse of T or F = 1/T.
Which aspect of this process do you require help with?