Tilapia generally holds up pretty well, so just oil it up well, don't move it until it unsticks on it's own. If you're afraid of stick-age, you can use aluminum foil under the fish. If you are successful there should still be grill marks. Also, I have seen a pannini guy use parchment paper slathered with butter or oil (to prevent it from just starting on fire) which worked very well for leaving grill marks. The paper method would obviously not be recommended for use on a real grill.