Why is my tablet storage almost full?
If you go to storage in settings, you can see a basic breakdown, I'm guessing apps wont be the biggest, however look to see if pictures and videos; audio; downloads; cache or misc. is taking up a lot of space and click on the relevant entry to have further actions (clearing cache, deleting downloads, etc..).

If this option isn't available in settings (it is in aosp, but may not be in touchwiz), try searching the play store for a comparable app (see @user284oo6).