Select the Crop tool from the button bar, or hit the "C" key on the keyboard. You can see the crop tool settings along the top. For calendar contest submissions, fill in the settings as shown here.
Now you can click and drag the crop tool to select the part of your image that you want to show. You can click and drag the corners of this selection box to reposition, resize, or rotate it as needed (I rotated mine slightly in this example). If you mess up, just hit the "Esc" key...