how to buy apps on my iPhone

You should be able to add the number from your visa debit card to your Apple ID. From settings on the iPhone touch the top panethat has your name. The third option once you do this is payment & delivery and there you can add the card number.

The other option you have is to find a store that sells iTunes cards or from the online Apple Store you can buy gift cards and email them to yourself

i already done that adding my card but still digs not work. otherwise can you give me some sites that offer gift cards?

As you're in Rarotonga, I'd try the NZ Apple site first

https://www.apple.com/nz/shop/gift-cards
https://www.apple.com/au/shop/gift-cards/itunes-electronic

i have installed the app, apple store. now can you please tell me hours do i start purchasing the apps i like?

please can you create a step?

Open the App Store app and then you can search  by pressing the magnifying glass in the right bottom corner, and go from there.
Once you have found an app you want, you will see a button which may be labelled "get" (this means it's generally free but may contain in-app purchases or upgrades that you have to pay for) or the button may have a price on it. Assuming you now have purchased credit, buying an app will take money from your Apple account.

Have you tried to add your Paypal account to your Apple ID?

https://www.businessinsider.com/how-to-add-paypal-to-apple-pay