You have to know the sampling frequency, which is the reciprocal of the difference between your time increments. I'm assuming that your time instants are evenly spaced. I'll assume you know your sampling frequency, but you can get if from your time vector if you do not.
Here is how to create a frequency vector.
t = 0:1/1000:1-1/1000;
x = cos(2*pi*100*t);
xdft = fft(x);
xdft = xdft(1:length(x)/2+1); % even length signal
Fs = 1000;
df = Fs/length(x);
freqvec = 0:df:Fs/2;
plot(freqvec,abs(xdft))
If you have the Signal Processing Toolbox, use periodogram() that will output a meaningful frequency vector for you.
