The honest way is going out into the world and finding wild horses to lasso and tame. This is fairly simple. Just calm a horse, mount it, and spin the left stick counter-clockwise so that it doesn’t buck you off. The dishonest way involves stealing horses that are already tamed. Either way, once you have a horse in your possession, travel to the horse shoe on the map and sell it. Stolen horses will earn less than ones you’ve acquired legitimately, so keep this in mind.