Do you mean the horse you are looking to buy or about horse related stuff in general? If you mean the horse you are buying them what @derekkeynes
said is perfect! If you mean horse related things then I would know about the horses bodies and health. Try learning about different illnesses that are common and what to do in case of them. Learning how to treat abscesses is also very good to know, as well as wrapping your horses legs. Just little things that can make your horse happier and healthier! Hope this helps!