From a generic point of view the FFT operation input and output data are Complex. However since time signals are usually Real (like in your case), there is also a version of the FFT VI that takes Real input and produces Complex output data. Likewise there is a Inverse FFT (IFFT) VI that takes the Complex spectrum data as input and returns Real time signal data.
In your case you should therefore use the Real FFT.vi located in the same palette.
The output of your FFT being complex, you probably just want to display the Power Spectrum. To do so, convert your complex output to "magnitude" using the Complex to Polar primitive (numeric>>complex palette), and square the "r" output to convert your magnitude r to a power signal.
Note that there are additional scalin
g operations that are needed to get the correct result. So why not using the high-level VI located in your Analyze>>Waveform Measurement palette? The VI takes Real values waveform input and takes care of all needed scalings for you.
