how download free app,if requires bank accaunt

You need to set up your Apple ID.

See this support page to get that done.

https://appleid.apple.com/cgi-bin/WebObjects/MyAppleId.woa/

Your Apple ID is easiest to use if you have payment information like your credit card entered. If you want to download a free app you will be asked for your password but you will not be charged anything for a free app.

You will be charged for apps that have a fee.

There is a way to set up an Apple ID without entering payment information. Read carefully through the support pages to find that info. As I said previously though, it is more convenient to have payment information stored in your Apple ID account.