Could be a number of Factors to take into account:
1). Videos
2). Pictures
3). Temp Data
4). Cache
Simply go to your Storage area and it will show you what is accumulated in each pertaining folder. For example, it may look like this:
Pictures - 500 MB
Video - 7 GB
etc.
Also, iCloud is good if you want to spend a little cash.
You say you have 12 GB Capacity. I am assuming your songs is what is taking up the brunt of it, but it shouldn't be anywhere close to 12 GB because having 12 GB in your phone is like having a bedroom that is by 100' - 100' feet in each diameter. That is the physical sum in life formulation, lol! 
Hope you sort it out!
