The problem you face is finding the schoolmaster horse which you need to each you how to ride. Most privately owned horses simply don't have the patience to teach a clumsy novice how to ride.
Secondly the rider training is a skilled art and not all riders have the ability. If you are taught incorrectly at the beginning then that bad schooling will last throughout your life.
To learn you need the placid horse, the skilled rider trainer and the flat fenced sandy arena. There are no short cuts.
Approach the BHS find the names and addresses of local register riding schools
Approach the BHS find the address of the secretary of the local riding club
- then phone them up and ask them if one of their members can help you.
Then go to your local Countrywide Stores and stick a postcard up on the board and say that you are willing to pay and help with a horse in return for some lessons. You'd better learn how to muck out.
But please make sure you have a good riding hat, a pair of riding boots, some riding gloves and an elasticated waist band. And don't make too many promises about what you will and won't do, until you have met with and sat on the horse.
But I warn you. I know over in my part of the UK, of an excellent riding instructor, who owns a suitable horse, and has an Olympic sized sandy arena -but she would not allow you near her yard because you would be a liability and she has no insurance cover for teaching a novice on her premises. And that is your fundamental problem.
Of course,you could always save up and go trekking in the Black Mountains of Wales - where you get to ride for an hour or so on a dobbin of a pony and you don't need to know how to ride.
Oh, I forgot. My own mare would have you off in about 3 minutes.
Best of luck - but be careful.