Hi, I understand complete what you are saying. However, your parents may realise you are shy and think that they are doing the right thing by getting you stuck in, so to speak.
It's difficult being a teenager (I do just about remember it myself), but if you want to be in the world of adults then you need to get in there at some point. Why don't you ask your parents if you can leave it until after the summer holidays, in the meantime looking for something you would rather do than work in a shop on Saturday. When you are young, going in at the deep end with something like this, is something you will be able to handle much easier than if you were older, young people accept change much easier than older people. Also with regards to responsibility, your age will excuse you quite a bit from any responsibility and you should be shown the job and shadow someone for a while until you get the hang of it.
It will definitely stand you in good stead for the future, if only for enhancing or developing your people skills, but you need to choose your working environment carefully if you are shy. Hope this helps and good luck. And think of all the things you can buy for yourself when you get paid!!