Hi how can i buy more storage if im using a debit card?

Hello!

You will have to add the card to your Apple ID as a payment method. Below you will find a link on how to change your payment method for your Apple ID. You will also find a link below with information about buying more iCloud storage.

Change, add, or remove Apple ID payment methods: https://support.apple.com/en-us/HT201266

Buy more iCloud storage: https://support.apple.com/en-us/HT201318

Let me know if this helps!