Grab the fast Fourier transform library of your choice and throw it at your data; you will get a decomposition of the signal into its constituent frequencies. You can then take that data and visualize however you like. Spectrograms are particularly easy; you just need to plot the magnitude of each frequency component versus the frequency and time.
I've managed the FFT and received a double containing values from -1 to 1. Can you explain in more detail what "plot the magnitude of each frequency component versus the frequency and time" means and how you would code that part?
The standard tool for transforming time-domain signals like audio samples into a frequency domain information is the Fourier transform.