I need to buy more storage can you help me ?

To buy more iCloud storage, see instructions in Buy more iCloud storage - Apple Support

Note that is for iCloud storage; you cannot buy more storage on the phone itself. See the following for information about the difference: What's the difference between device storage and iCloud storage? - Apple Support

You cannot buy more storage. The device will always have the amount it came with. The only way to gain space is to delete photos, messages, apps, etc.. Be sure to save important photos and data to yur computer or external drive. Check these links...

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

Clear Other storage on your iPhone or iPad - Apple Support