how to buy or get iphone storage

You cannot expand the capacity of your iPhone.

About storage on your device and in iCloud - Apple Support

You'll need to know what's taking up all your space to decide how to proceed..

you may end up needing to delete stuff, you may wind up needing an iPhone with more storage..

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

If Photos is the big space killer for you

Set up and use iCloud Photos - Apple Support

Buy more iCloud storage - Apple Support

or you could transfer them to a computer and remove them from your phone..

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support

if you see 'other' taking up a bunch of space..

Clear Other storage on your iPhone or iPad - Apple Support