how do i buy more storage

After reading the support article that led you here, bearing in mind that the iPhone internal storage cannot be increased (you can just make room by deleting unused stuff), if you need more iCloud storage, start here:--> Upgrade to iCloud+ - Apple Support

Regards

Giulio

See the instructions in Upgrade to iCloud+ - Apple Support

Note that is for iCloud storage; you cannot buy more storage on the phone itself after the initial purchase of the phone.