Have you had riding lessons? If you have not then you need to focus on that before even thinking about training a horse to ride. If you don't have the skill-sets then you are only headed for disaster. However if you are a current rider with no horse care and training experience. That's different. I also would reccomend boarding (even if you buy a ready-to-ride horse) because you need to learn the ins and outs of proper nutrition along with the ability to spot problems such as injuries and illness.
Good luck to you!