iTunes took 20$ from my credit card
I want to know what for is payment of 20$ is made?
Well, what makes you think that iTunes took $20? Did you get an email? Did you see this on your credit card statement? Do you see anything in your purchase history?
If this was an email, it could possibly be a phishing attempt to get account information. If you check your credit card statement and there is no charge, or you check your iTunes purchase history and there is no history of a purchase, then it is probably a phishing attempt. Here are some support documents to look at to check for fraudulent messages, what to do if you see an unknown charge on your credit card account, and how to check your purchase history.
Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support
See your purchase history in the App Store or iTunes Store - Apple Support