Converting x-axis in terms of frequency

I'm having difficulty converting my x-axis. I have converted the
complex
numbers of ffted data (X) to real values by applying the conjugate.
My new data is Z which has been ffted and where the hamming window
has aslo been applied.

I'm having difficulty labelling and converting my x-axis. I need to
represent it in frequency terms.

Data = Z
Sample rate = 44100Hz
Total no. of frequencies/values of data = 88200
Length of signal = 2 seconds

I have also applied fftshift to the data (Z) so that my left half is
negative and the right half is positive.

How can I represent the x-axis in frequency terms?? I'm not sure what
coding to use.

If someone could also tell me how I will be able to get rid of
certain
frequencies e.g. I want everything below 20Hz and -20Hz to be cut
off/throw it
away??
.