When you put the main dial on "S" that is shutter priority. When in that mode you can move the command wheel and change the shutter speed (right side of camera upper right). The 1600 is probably the setting for the ISO, best guess. You need to go into the menu and adjust the ISO to something like 200 to 400 That represent the sensitivity response of the imager. 400 is for less light, and should give you faster shutter speeds if you are in 'AUTO' .
Go to Barnes and Nobel and find a book called "Digital Cameras for Dummies" really, that is the title. Go through that and go slow, it is kind of a funny learning curve. At first it doesn make sense and then all of a sudden things become real clear.