If you would like to erase something, here's how. The circle shown was made from the Circle tutorial.
In the vertical tool bar on the left side of the screen you will see a little eraser tool like shown. Click on this.
Now lets select how big our eraser is by clicking on a brush type in the brush pallet like shown. If you don't see the brushes, try going to Windows>Show Brushes.
Now just hold down the mouse button and erase your object