First read this page http://www.eduweb.com/portfolio/earthsystems/food/foodweb1.html
But wait...how are food chains formed? http://www.eduweb.com/portfolio/earthsystems/food/foodweb2.html
Take notes on energy pyramids and trophic levels
Next watch this video. https://www.youtube.com/watch?v=MGODmyXkkPU
Take notes on what a food web is.
How are food chains connected? http://www.eduweb.com/portfolio/earthsystems/food/foodweb3.html
Now go to this website and manipulate the food web to see what happens. http://www.eduweb.com/portfolio/earthsystems/food/foodweb4.html