there is a small amount has been deducted , i want to know why ?

Aliah,

It might be a temporary authorization hold. If so, it is temporary and will be removed. Look at the section "If you see a pending charge for a small amount" in here:

How apps, content, and subscriptions from Apple are billed - Apple Support

Aliah,

First find out if the SMS is really from your bank, and isn't juts a phishing scam. If the charge is really on your account, follow the steps in here:

If you see 'apple.com/bill' on your billing statement - Apple Support

Aliah,

If the charge is showing on your bank account, but not on Apple, you better contact the bank. They can find out whether there has been fraudulent use of your account.