Relationship between Fs (the Nyquist frequency), and the frequency used with a sine wave
The sampling frequency fs is 1/dt. Conceptually, if the time between samples is $T$, then you have $1/T$ samples per second, which is the sampling frequency.

Next, you need to make sure that frequency, the sine wave's frequency, is less than the Nyquist frequency, which is fs/2.

For interpreting the spectrum (FFT_intensity), please see the answer pointed to by Matt L.