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.
Certified Professional Instructor Certified LabVIEW Architect LabVIEW Champion
The problem seems to one of formatting the DFT result. The FFT returns a complex result, which can be displayed as magnitude/phase or real/imaginary. To get something analogous to Fourier series coefficients, use the real/imaginary format--the real part will be your cosine coefficient and the imaginary part will be your sine coefficient. You might have to worry about converting from RMS to peak by multiplying by sqrt(2) and perhaps another factor of 2 if you use a double-sided FFT. For an easy single-sided result on a waveform input, try the FFT Spectrum (Real/Im).vi in vi.lib\measure\maspectr.llb. If you prefer the array-based FFTs in 2dsp.llb, you will need to use the Polar to Complex and/or Complex to Re /Im math primitives to convert your results.