I can't make sense of the amplitude spectrum of a signal. Help please
A few suggestions and comments:


You calculate the frequency vector with F = [-N/2:N/2-1]/N;. This is wrong. You need to take the sampling frequency into account.
The way you organized your code, you don't have direct control over the sampling frequency. I recommend specifying both the time length and the sampling frequency explicitly in your code.
You need to set up your analysis so that you get high resolution in the frequency domain (so that you can resolve peaks that are very near to each other). The only way to achieve this is to sample for a long time. The more you oversample the signal (that is, the higher the Nyquist frequency is), the longer time you need to observe the signal. This is the reason you want to have explicit control over sampling rate and signal duration.
Do not limit the FFT size by choosing an explicit N, since that may lower your frequency resolution. If you want to set the FFT to a power of 2, then use nextpow2 or similar.