Grilling fish can be tricky -- they stick to the grill a lot. So minimize this by making sure your grill is very clean, very hot, and very well oiled. Do this by soaking a paper towel in canola or some other inexpensive oil, then wiping down the grill with it just before putting the fish down.
As for the fish itself, it should get a coating of a tastier oil such as olive or sesame oil. This helps prevent it from sticking to the grill and helps the seasonings adhere to the fish.