You don’t provide much to work with, but assuming you want to take the fft of ‘y’ that corresponds to uniform sampling times in ‘t’:
Ts = t(2)-t(1); % Sampling Interval
Fs = 1/Ts; % Sampling Frequency
Fn = Fs/2; % Nyquist Frequency
Ly = length(y);
yfft = fft(y)/Ly;
Fv = linspace(0, 1, fix(Ly/2)+1)*Fn; % Frequency Vector
Iv = 1:length(Fv); % Index Vector
figure(1)
plot(Fv, abs(yfft(Iv)))
grid
