To erase something, press the Eraser and use the mouse to click and drag over whatever you want to erase. To use the pen again, press the Pen icon next to the eraser.
AT LONG LAST! I AM BACK! Here is a simple drawing program I made in a few hours (okay, it wasn't simple for me :3) Have fun! Question: How can one make an undo and redo button? Do I have to necessarily make a list?