If it is not your iCloud storage as the issue as Ariza16 outlined - checking the internal storage available on your iPhone, go to Settings - General - Storage & iCloud Usage - then select Manage Storage in Storage section (not the iCloud section)
You will see how much internal storage is used and also available on the iPhone itself. Also a list of all your apps and how much storage they are taking up on the iPhone.
If you Tap on an app, you will have the opportunity to delete it from this are also.
Apps on the iPhone create large invisible cache files. The iPhone doesn't allow us to delete or clear those files directly, we have to delete the entire app, then re download the app to clear it all out.