4. Choose the storage plan that is right for you and then click "Buy." You will then follow the onscreen prompts such as entering the password for your acount. Please make sure you discuss your storage options with your parents before upgrading. You may purchase additional storage using a valid credit card or iTunes gift card. The amount will be deducted each month. You can also downgrade at any time.
For more information on their iCloud storage plans see Apple's website.