Determine frequency of audio signal in Audacity

I need to find the frequency range of musical audio signal (mp3 files), and also to get a graph of the frequencies, to determine frequency allocation, to see what frequency range contain the maximum audio information. Windows 7.

Plot Spectrum:

what settings should I select in Plot Spectrum for this?

See the Manual. Choose appropriate settings for your file. You may want to change to Log axis which is a better representation of how we hear audio, with lower frequencies given more width.