I guess you want to measure the frequency (mhz?)? Though I don't think the mic or the ADC (analog to Digital converter) in the iPad is able to work in the Mega Hertz range, probably up 20-40 kilo Hertz.
Look up frequency analasys, FFT and so on, should give you some idea what you need to do.
You need to use the divide and conquer approach.
(FluBBa's confusion comes because mHz stands for "milliHertz", or 1/1000 of a Hz. For audio frequencies you would use plain Hz or KHz [kiloHertz]. FluBBa [reasonably] assumed you meant MHz or MegaHz, which is millions of Hz, usually used for RF signals and way beyond the iPads capability.)
First learn how to write a basic app that displays a number.
Next read the microphone and determine when there is a signal.
Lastly, perform an FFT to turn the signal samples into a set of frequencies.
I suggest you get something like MatLab to experiment with the signal processing.
This might help you get started: