how can i buy more storage?

You cannot buy more storage. Your phone will always have the same amount of storage it came with. You can buy space in iCloud, Apple's synching service. The only way you can create more space is to delete photos, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. Check these links for more explanation...

How to check the storage on your iPhone, iPad, and iPod touch - Apple Support

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

Where? You can’t buy more storage on your device, but you can buy more storage in the cloud. Settings> Apple ID> iCloud> Mange Storage> Change Storage Plan should help. Unfortunately, you can’t buy more storage on your device (it’s built-in).