You need one of two things:
1) The official or Samsung-sanctioned wireless charger back cover, which adds the necessary charging coils to the phone. This is the safest way to add wireless charging. Problem is that these backs usually add bulk to your phone, which means you probably wouldn't be able to continue to use your Otterbox case.
2) Get a 'coil card', which is basically a 3rd party add-on that sticks over the battery and under your current battery cover and plugs into your battery charging pins. This doesn't add as much bulk, but since it's an add-on and it's basically a metal coil that's being put on top of critical hardware components, it's not the safest, most recommendable way of doing it. However, this should allow you to keep using your Otterbox case. BUT since these add-ons aren't top notch, there is a possibility that they might not be able to receive the signal from the wireless charger if you don't remove the Otterbox case while charging; YMMV.