When I turn on the faucet, clean water leaks in to the dishwasher. How do I fix it?

Do you mean that water going down the drain goes into the diswasher?

How is the dishwasher hooked up? It makes no sense that water would run into the dishwasher unless you don't have an air gap installed. Here is one: Mueller 995-000 Dishwasher Air Gap_Speedy Delivery_866-275-7383 (http://www.google.com/products/catalog?q=air+gap+for+dishwasher&um=1&ie=UTF-8&tbm=shop&cid=4304381364184685455&sa=X&ei=XzDnT_i0CoKQ8wTpmYjNAQ&ved=0CJYBEPMCMAI)
And here is a video: The Diswasher Air gap - how to clean it and why it's important - YouTube (http://www.youtube.com/watch?v=rmG2JXQL858)
If water is coming from the drain you need to install this air gap which costs $3.00.