deltaFreq = sampleFreq / numSamples
The first numSamples / 2 bins of the FFT are the positive side of the spectrum. Their frequency values are spaced deltaFreq apart and run from 0 to deltaFreq * (numSamples / 2 - 1).
Since you’re probably taking the FFT of a signal with only real values, the spectrum is even and you can ignore the negative frequencies represented by bins numSamples / 2 to numSamples - 1.