The Stern-Gerlach experiment is imo the best way to learn the basic idea of quantum mechanics as derived from a reasonably simple experiment. Much better than the double-slit experiment which is more confusing and about which there is a lot of terribly incorrect discussion online.

I would recommend another source with some nice diagrams though, like hyperphysics, or the opening chapter of Modern Quantum Mechanics by Sakurai (although it's a graduate level book, the opening chapter is simple enough).