As far as I know, rect is not a MATLAB function. Perhaps it is one you defined and saved in an m file. Assuming this is the case this is the typical pattern for plotting some function.
% define the dependent variable, in this case t over the range of interest
t = linspace(0,10); % for example from 0 to 10, you can put in the range you wante
% evaluate the function, assume function is vectorized so it can accept a
% vector input
x(t) = |t| * rect((t - 1)/3);
% plot the function
plot(t,x)
title('my function')
xlabel('time [s]'); % put in what you want for x axis label
ylabel('my functions value'); %put in what you want for the y axis label
regarding performing fft refer to the documentation
type doc fft on the command line
