Paella naturally sticks to the bottom because of the starches in the rice and the creation of a prized crust called socarrat. When you cook paella, you should avoid stirring the rice after the broth is added, which allows those starches to settle, toast, and caramelize against the metal.
To make cleanup much easier next time, ensure your heat source is evenly distributed and that you aren't cooking it too hot. If it sticks too hard, letting the pan soak in warm water with a little dish soap for twenty minutes will loosen everything right up.