How to plot the spectrum of a signl of "ones"

I need to plot the frequency spectrum of an "ones" function like in this image: https://screenshots.firefox.com/kPEJ1moZvdAtG7To/cld.pt

First of all I am not being able to plot a correct graph.

Second, in the spectrum imagem the first frequency "S" is okay, but where does the other frequencies come from?

Thanks, in advance!

[Naman Bhaia]

Can you post the picture here as a comment and also show what you have tried? This way anoyone trying to solve this will have a starting point.