How do I pay for more storage on my phone

Hello hali93,

If you happen to be referring to the internal storage of the iPhone then this cannot be upgraded after the device is purchased.

If you're looking to purchase additional iCloud storage then you may find the needed steps and info on that here: Upgrade to iCloud+ - Apple Support

How to upgrade on your iPhone, iPad, or iPod touch

1. Go to Settings, then tap your name. 

2. Tap iCloud, then tap Manage Account Storage or Manage Storage. Or scroll down and tap Upgrade to iCloud+.

3. Tap Buy More Storage or Change Storage Plan.

4. Choose a plan and follow the onscreen instructions

This link can assist with providing clarification on the difference between device/internal storage and iCloud storage, if needed: What's the difference between device storage and iCloud storage? - Apple Support

Cheers.