you can make one on your computer by getting an application free here: http://www.sciencefair-projects.org/solarsystem/solarsystem.html
or do this for a real one:
Materials:
(All of these materials can be found at a local craft store)
1 big styrofoam ball for the sun (about 8 inches in diameter).
9 smaller styrofoam balls of different sizes to match the planets.
10 wooden dowels (read the instructions below to determine what size).
1 box (about 3 x 3 x 3 feet).
black thread.
pins.
clay (a small amount).
glue.
paint (black for the dowels and box; yellow for the sun & stars; other colors for the planets).
construction paper (red, white, or yellow for rings around planets).
1 cork.
1. First, find a good box. It should be big enough inside to hold 9 planets revolving around the sun.
2. Tape the box shut on three sides, leaving one side open. Remove the flaps of the box from the side that is open.
3. Use black paint or construction paper to cover the inside of the box. This will be the universe. Paint small yellow or white stars as a background for your universe.
4. Get styrofoam balls for the sun and planets. Make sure the sizes are proportional to the real solar system, but on a much smaller scale. Paint the styrofoam sun yellow. Paint the planets whatever color you want, but try to remember the color of real planets (blue for Earth, red for Mars, etc.).
5. Paint all of the dowels black, this way they will blend in with the background of the universe box.
6. Cut 2 dowels, a short one and a long one, and poke them into the cork. Use a nail to make the holes if the cork is too tough. Glue the dowels so they stay in better. It should look like this:
7. Make sure the total length is long enough to fit snugly inside the box and leave about 2 inches on the top dowel so it sticks through the box. By having this stick through the top, you can turn the dowel and make your planets rotate around the sun.
8. Now, slide the sun up through the bottom, longer end of the dowel.
9. Poke a hole through the top of the box. From the inside of the box, slide the top end of the dowel through the hole. Secure the bottom of the dowel with clay. Glue the clay to the box if it moves around too much. So far, it should look like this:
10. Now you're ready to insert the planets. Cut the remaining 9 dowels at different lengths. These will be the distance from the sun. If you make the lengths equal, all the planets will crash into each other.
11. Insert all 9 dowels into the cork, so they form a "pinwheel." Glue the dowels for a stronger hold. Here's what it would look like from the top:
12. Take the thread and tie it on the end of these "pinwheel" dowels. Tie the other end of the thread to a pin. Poke the pin into the styrofoam planets. Make sure to place the planets in the correct order based on distance from the sun. It should now look something like the picture below. (Note: In these pictures the dowels are white so you can see them, but your dowels are supposed to be painted black).
13. Cut construction paper to make rings around the planets that need them. Secure the rings to the planets with pins or glue.
14. By twisting the dowel that is sticking out through the top of the box, you can bring your solar system to life.
to help you scale your project I'v attached a link: http://www.exploratorium.edu/ronh/solar_system/