Why is my iPhone storage space used so high?

I have a 16GB iPhone 4s. I have no songs or videos on it. I have about 32 apps and 500 pictures. My storage space used is at 13.2 GB. Why is that so high? That is almost all of my available data?

How can I bring this down? I have a friend with the same phone, same number of pictures and about the same number of apps (maybe a few less) and his storage is only about 4GB used.

Sadf