Q: How to download music from iTunes by using debit card

If debit cards are an acceptable form of payment in your countries iTunes store, then enter it in your account information, find what you want, buy it.

Payment methods you can use in the iTunes Store, App ...

Looking at your user name, I am guessing you have Indian connections.   If so, and you are resident in India, as far as I know, Indian Debit cards are not acceptable in iTunes or the App Store.