Extracting frequencies from FFT
Assuming your 512 samples of the signal are taken at a sampling freqeuncy $f_s$, then the resulting 512 FFT coefficients correspond to frequencies:

0, $f_s/512$, $2 f_s/512$, $\ldots$, $511 f_s/512$

Since you are dealing with discrete-time signals, Fourier transforms are periodic, and FFT is no exception.

Therefore you can think of your last coefficient as corresponding also to frequency $511 f_s/512 = (511-512) f_s/512 = -1 f_s/512$.

The same applies to the second to last coefficient, and so on. This is the mirroring commented by Daniel Hicks.

Also, if you are transforming a real signal, then all your information is contained in the first 256 FFT coefficients. The rest are simply complex conjugates of the first coefficients.