Use a trim removal tool. (A fork will work somewhat, in a pinch.)
See if you can search for pictures of the trim off to see what you're dealing with: there are around 5 white plugs that snap into the car. They come out with the trim. You can't apply pulling pressure on the trim too far away from the plug, or you'll break the black trim. But when you start applying pressure near the plug, you'll notice the trim isn't bending much, and with additional pressure the plug will pop out.
I'll be honestly, its not a pleasant experience.