Well, that depends on what exactly it is that you want. If you want a power spectrum of the input data, there's a VI in 2dsp.llb called "Power Spectrum.vi". If, on the other hand, you want the amplitude and/or phase spectrum of the input signal, the library 0measdsp.llb contains the VI "Amplitude and Phase Spectrum.vi".
Which one you use depends on what you need. If you're not sure what you need, start with the Amplitude and Phase Spectrum.vi.
Mike...