I spent my work experience at HMS Gosport, the Navys mech and air engineering school. For a 15 year old it was pretty awesome- two days were spent crawling round sea-kings, helping to replace parts under supervision; one day was spent on the HMS Ark Royal, one day was spent on a nuclear sub engine simulator... working out how to divert power in case of failures etc, and another day was spent learning about the physics of the Rolls Royce Olympus engine (which was on Concorde).
As people have said, try and go somewhere that you think you might actually learn something. Going to a store and ******* around is a waste of your time and theirs... as someone suggested, go to a garage where they would be willing to teach you a few basics and it could be soem good training for when you get your own car - itll save you money if you can do all that sort of basic stuff yourself!
NO SWEARING.
Gilly.