You can find better introductions to category theory available on the net for free. Try searching for Lambert Meertens, Marten Fokkinga, and Jaap Van Oosten, for example. Or Barr and Wells, Triples, Toposes, and Theories. Or Asperti and Longo. Or watch Eugenia Cheng's videos on YouTube, which are fantastic.
But if you want to buy a book, get Barr and Wells, Category Theory for Computing Science. Unfortunately, you have to order it directly from the University of Montreal. It's a great book, by far the best intro to category theory available, *way* better than this! Then, after that, you can read MacLane...