How do I get more storage on my phone
More storage
You cannot add more memory to the phone, that leaves the alternative of managing what you have to better effect.
That might be achieved by deleting apps or content, or by offloading some of that content to iCloud (you'd almost certainly need to subscribe to one of the paid iCloud plans to do the latter).
See the following articles:
How to check the storage on your iPhone, iPad, and iPod touch - Apple Support
Delete songs and videos from your iCloud Music Library - Apple Support