Your operating system.
What kind of board you have. If it's a Mini, LilyPad or other board that requires extra wiring, include a photo of your circuit, if possible.
Whether or not you were ever able to upload to the board. If so, what were you doing with the board before / when it stopped working, and what software have you recently added or removed from your computer?
The messages displayed when you try to upload with verbose output enabled. To do this, hold down the shift key while clicking on the upload button in the toolbar.