Since you said you "barely know anything about the program yet", my first recommendation is to work through the free, roughly two hour long, MATLAB Onramp tutorial. The goal of this tutorial is to teach you the basics of working with MATLAB.
As for the hint you received, I'd be wary of using str2num. Instead I recommend str2double. str2num evaluates the text you pass into it, which would mean if your user entered the text 'exit' at your prompting MATLAB would exit because the exit function was called. str2double doesn't do that. You would have to use a slightly different function to detect the user entering something that's not a number, so I'll give an alternate hint: isnan instead of isempty.
