Convert i/q data to power measurement (e.g. db)
Typically, you take the I &amp; Q signals and perform a Fast Fourier Transform (FFT). This gives you a spectrum (or waterfall) which shows you the relative strength of signals.
Don't forget, dB is a logarithmic scale. You will need to change the magnitude (y) scale after the FFT, but before displaying it on screen. Then you will need a method of selecting a signal on the display, to be further processed and turned into audio.
If this sounds a bit too complex, you can always try reading through the github source code for gqrx. You can check out the SDRSharp source code here.