I want to do my works experience in HMV,?

Just go into your nearest store and ask whether they take work experience students, as not all employers do. If they say yes ask them what you need to do to apply. They will probably say write to the store manager. Write him or her a letter giving the dates you want to go there (give them a couple of options on dates if you can) and let them know a little about you and what you're studying and why you'd like to work there.