To select part of an image, you simply place the cursor at a desired location in the image and drag the mouse while holding the left mouse button down. While dragging, you will see the selected area delimited by an inverted rectangle. To finish selecting the area, simply release the left mouse button.
To select an area from the center out, hold the SHIFT key down, press the left mouse button at the place you want to be the center of the selection, and drag the mouse to a corner. Then release the mouse button.
See also: View Window - Mouse for more mouse button information.
You can also use the keyboard to select, size and move a selection rectangle. See View Window - Keyboard for more information.