Gravity, in the most fundamental sense, is not really a force between objects with mass. Einstein discovered that gravity is an effect on space itself! The mass of the sun, for example, bends the space around it, so that the Earth travels in an ellipse rather than a straight line (as it would in a "flat" space).You can make a model of this with a large rubber sheet (or a springy fabric). Pull the sheet taught, and put a heavy weight in the center. This will bend the rubber, so that when you roll a marble on the sheet, it will "orbit" around the "sun" in the center.

Now, to make the model more like a black hole: cut a circular hole in the center of the sheet, and pull the edges of the hole down. You can still get things to orbit the hole (as things can orbit real black holes), and there are also trajectories that will spiral in to the black hole, never to be seen again.

If you want to go one step further, instead of just doing a Swartzchild black hole (the non-rotating kind) you could do a rotating one (a Kerrblack hole). This kind of black hole actually drags the space with it as it rotates. (Imagine the hole in the rubber sheet rotating with the edges of the sheet fixed - this would also alter the path of the marbles on the sheet - and if you drew a grid on the sheet you would also have a picture of how the space was deforming.)

I imagine you're expected to do more than just a demonstration for your science project. You'll want to have some sort of testable hypothesis as well. Perhaps with the model, you could test which types of marble trajectories escape the black hole and which types will spiral in. Does the angle you throw the marble at matter? What about the speed?

I hope this is helpful. Don't limit yourself to these ideas. Have fun & good luck on your project!