What a great question! Scientists believe that the Earth was created when the Universe was created in an event called the "Big Bang" (not the TV show!).
Here are some links you might find interesting. Your teacher should have a password sheet for you or you can find them beside my computer. All of these resources can be found on the VLC.
There's also some great information about the Earth in general - like how mountains and lakes and things were created over time, by earthquakes, volcanoes and weather events.
