How can I know/represent the frequency of my signal on a plot after applying fft function to my signal?

I am collecting data from a sensor to MATLAB at 55Hz of sampling frequency.I want to know the actual frequency.Because the fft function does not seem like showing the frequency. My code:

 clc
 clear all
 a = arduino('COM5','Mega2560')

 fs=55;
 Ts=1/fs;
 t=0:Ts:6;
 count=0

 for c=0:Ts:6
    count=count+1;  

   z1(count)=readVoltage(a,'A3');
   y1(count)=readVoltage(a,'A4');
   x1(count)=readVoltage(a,'A5');

   x2(count)=readVoltage(a,'A0');
   y2(count)=readVoltage(a,'A1');
   z2(count)=readVoltage(a,'A2');

 end

 j=fft(z2);
 plot(abs(j))

Can any one help me with that, I will be thankful