When I connect my iPhone to my pc, it shows that the internal storage of the phone is almost full, however when I click the subfolders, there are no photos or videos inside the folders to delete. How can I delete what I can't find?
If you are talking about using windows explorer to delete files, that would be of no help. Windows Explorer can ONLY see photos and videos. It treats the phone as a camera and can only see such.
Use iTunes to manage the storage of the device. It will even give you a breakdown of whats using the most on the phone.
Alternatively you can go to
Settings -> General -> Usage -> Manage Storage on the phone itself and delete some stuff from there.