I need more storage on my iPhone

No way to add more storage to the iPhone, so you will have to start deleting things from the iPhone to make more room.

If you have a lot of photos, you might consider moving them all to iCloud......for a fee of course.

Transfer photos and videos from your iPhone, iPad, or iPod ...Official Apple Support › en-us

https://www.businessinsider.com/how-to-free-up-space-on-iphone

The only way you can gain more space is to delete photo, messages, apps, etc.. Be sure to save important photos and data to your computer or an external drive. this video may help...