Plotting differents square waves

Hello!!

I need to plot differents square waves changing the frecuency for later on compute the fft and see how it changes. The problem is that I dont know how to plot a function that from 0-Tc/2 is 1 and from Tc/2-Tc is -1. One that I clear that, I need to compute another function that # 1 for 0-Tc/4 # -1 for Tc/4-Tc/2 # 1 for Tc/2-3Tc/4 # -1 for 3Tc/4-Tc

and then so on, to see how the frecuency change affect to the fft. So I have to be able to compute the fft of the function, so I think that I cant define just vector and plot them. I am a little bit lost, I will appreciate any help Thanks in advance, NIcolas