If you are serious about this you need to start reading about the subject. You need to learn the physiology of the horse, tack (saddle and all the stuff one sticks on the horse), and beginner horsemanship. There are lots of books and info available. Most tack stores have a good choice of material for sale.
Then, if you are still interested, it would be a very wise decision to find an instructor who could give you lessons. There is so much more to riding than just getting on and going. Just getting on a horse can be work...unless you are young and spry. And I would make an attempt to get into a fitness program. Larger people can develop muscle tone and strength and that will help a lot.
Good luck to you.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger