You are getting mirror images. except that matlab's FFT results are different from what you have read in your books. Normally FFT algorithm results are from -Fs/2 to +Fs/2, where Fs is the sampling frequency. Matlab's output gives from 0 to Fs. Saying that, if you want to see the plot from -Fs/2 to Fs/2, use the command fftshift.
Your peak at 0 frequency means there is a large DC value. To remove it, find the mean of x and subtract it from each and every value of x.
This should remove the mean value.
If your x has the senor's resonance, then it should be visible as soon as you have removed the mean. if not, provide the x data in the question, so that people can check it and give you better answers.