How can I buy more storage?

Remember that you will not get more space internally on your iPhone by purchasing more space in iCloud. Apps, photos, music etc. will still take up space in your iPhone's memory. iCloud will just be extra space where you can have all your stuff backed up.

You can read more specifically what the difference is between iCloud and Internal Storage here:

What's the difference between device storage and iCloud storage? - Apple Support

If you want to buy more space in iCloud and upgrade to iCloud+ see here:

Upgrade to iCloud+ - Apple Support

The support document just above your name pretty well sums things up.

You can't create more storage on the iPhone....that number is fixed.

You can buy more storage on iCloud.