How to Fourier Transform (basic understanding)
If you want to plot the magnitude of your data you need to calculate first the absolute value as the output of the fft is complex.

z = fft(y);
z_mag=abs(z); % you could also log-tranform the data for db-scale


To get the right x-Axis you need however the sampling rate. Lets assume that the sampling rate at which the data is recored is f_sampling(some number);

Fs=f_sampling;
n=length(z_mag)
freqHz = (0:1:n-1)*Fs/N;


Keep in mind that this is a double-sided spectrum.