How do I get more storage on my iPhone 6

You don't, storage is a fixed size that can't be increased.

Here are some helpful articles that can guide you on how to free up storage space on your phone.

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

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

Manage your photo and video storage

Manage your photo and video storage - Apple Support

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

You delete any items you no longer need (if they are photos be sure to delete them from the Recently Deleted folder in Photos) and if you have Recommendations when you check your storage as described in the article you came here from you follow them. You cannot add physical storage.