Where do I go to pay for more storage

Hello deboMylife, If you mean Physical memory the only way is to buy another iPhone with more memory. iPhone memory is not upgradable. If you mean Backup Storage in iCloud then on you iPhone Settings > iCloud > Manage Account Storage and click. Choose an option.

iCloud+ plans and pricing - Apple Support

Bearing aware that an iPhone "internal storage" cannot be upgraded (you can just make room by deleting unused stuff), if you need more iCloud storage, use the instructions provided in the related article that led you here.

You can better understand the difference in this support article:--> What's the difference between device storage and iCloud storage? - Apple Support

Thanks and regards

Giulio