If you want to keep a perfect circle or square, hold the shift key while dragging out the marquee.

If you delete the selection and you don't like it you can hold the control key and click Z. Control Z is undo.

Control C wil copy whtever is selected to the clipboard and can be pasted in another area. It can also be pasted in most other progams.

Control V wil paste whtever is selected to another area. It will also paste what has been copied into most other progams.

Control N will create a new document.