analysis of answer of FFT function of Math.net

Question

I had converted a 3gp file to bytes array, after transformed it in a complex class (add "0" in the imaginary part) and finally using the Foward Fourier function in this array. I need to descompose in waves frequencies through FFT. I used the Math.net library for C# (In Xamarin) but i don't know how use the Fourier function.

public Complex[] CalculoFFTComplejo(in string pathFile)
    {
        var bytes = File.ReadAllBytes(pathFile);
        var valoresFinales = TransformarComplejos(in bytes, bytes.Length);
        Fourier.Forward(valoresFinales, FourierOptions.Default);
        return valoresFinales;
    }
private Complex[] TransformarComplejos(in byte[] valores,in int tamanio)
    {
        var arregloComplejo = new Complex[tamanio];
        var indice = 0;
        foreach (var valor in valores)
        {
            arregloComplejo[indice] = new Complex(valor,0);
            indice++;
        }
        return arregloComplejo;
    }

and now, how do I analysis this result to separate in spectral graph?, How do I know which part of the array shows the results for the different wave frequencies?