I would definately find somewhere to learn some basic stable management before thinking about getting your own, or even taking on a share?
Where is your stable management at the moment? Are you at the stage where you could not even put on a headcollar properly, or more at the stage where you are interested in things like feeding/fitting tack correctly?
Would www.bhs.org.uk/training-and-qualifications/exams-and-qualifications/horse-owners-certificate be of interest to you?
You could also look for a stable which offers BHS progressive tests, which will help you work through stable management in a structured way.
If you were looking to take on a share, on your share days you are usually unsupervised or lightly supervised, so at an owner would probably expect you to:
-Be able to catch the horse/pony from the field and turn them back out.
-Be confident generally handling the horse, tying up, and leading.
-Know how to put a rug on a horse/pony and when different rugs are needed.
-Be able to tack up for yourself, and have some idea of if the tack is fitting correctly or not (this doesn't mean expert knowledge, just being able to see if something is obviously wrong).
-Be confident grooming the horse/pony, including picking out their feet.
-Some basic knowledge of feeds, so you could feed the horse/pony if needed. Also how to fill and tie up a haynet safely.
-Some basic first aid knowlege, and the symptoms of problems like colic and lamminitis. Know when you need to call the owner and when you need to call the vet.
-How to recognise if the horse/pony is lame.
-It would be good if you could tell when the horse/pony needs shoeing, although not 100% esential for a share, you would obviously need to know this when you have your own.
-You would also be expected to muck out the stable on the days when you shared the horse (although the owner will probably explain exactly how they want things done).
-You may be expected to clean tack, possibly after each use.
-You may need to put on any boots/bandages needed for exercise, although the owner will probably show you how to do this if it is anything unusual.
If you have your own there are other things you will need to keep track of, like dentistry, worming, injections etc.
Some share owners may be happy to show you things if you are inexperienced, but in general owners take on sharers because they are short on time, and want someone they can leave to get on with it. I also think on full livery it is still good to have some knowledge, so you are not just blindly accepting the things the livery owner tells you.
I hope the list doesn't sound too daunting. Learning about stable management is pretty fun if you really love horses. If you're at a good riding stables they should be happy for you to help out, and will be happy to answer all your questions and show you how things are done.