how to buy app store code online?

When you log into iTunes Store on the computer, you will see a link for Send iTunes Gifts. You determine there amount from there. However, understand that iTunes gift cards that you purchase are only valid in the country where you purchase them, so you cannot purchase a gift card for someone that does not live in your country.

You could, but you cannot purchase the iTunes gift card online without a credit card. They can only be purchased through your iTunes account. You could purchase one at a local store and load it into your account by redeeming the code.

An iTunes Gift Card can be used to purchase music or apps.