How to get main frequency from digital signal
Generally, yes, you should perform FFT and take the highest peak from the first half of the transformed signal. However, if the frequency your are looking for is constant (or close to constant), i.e. doesn't change (much) throughout the observation time, I would recommend using Welch's method instead of a simple FFT.
If the accuracy is not good enough you can perform interpolation around the peak, or on the entire signal using zero-padding.