Go to "Applications / Utilities" and open Grab, then select Capture / Screen and save the image to your desktop. Then open the file and print from Preview or open "iPhoto / File" and "Import to Library" then edit and print. You can also click-hold and drag a copy of Grab's (or any other app's) icon to your dock for easy access.
Don't need to open
GRAB as there is a keyboard combination, in-fact there are two you can use.
For full screen grab use:
Command (Apple Key the one either side of the space bar!), Shift (Up Arrow), 3
Sequence should be:
Hold down command key with left thumb, use left little finger to simultaneously press shift key then use index finger to press number three key ALL AT ONCE !
If you press the number 4 instead of 3 the mouse curser will change to cross hairs which you can position over anywhere on the monitor (You will also see two numbers which represent the pixel size of the clip). Press and hold mouse down over area to be copied.
In both you will see the captured image on your desktop.
As default the image is a PNG file.
By the way you can also invoke the Grab application in just about ANY application without launching it independently.
Just click on the name of the open application in the menu bar at the top of your screen, mouse down to the services menu and you will see GRAB in the pop up menu.
Grab generates TIFF files by default.
Open either image in Preview.
Click on File in menu bar go to Save As and in the Format box choose from a variety of popular image formats such as JPG/GIG/PDF and PHOTOSHOP compatible and the format will be converted and re -saved in you chosen folder.
You can then open this image in your photo editor such as iPhoto for limited editing/correction, or Photoshop Elements (Good Buy!) or Graphic Converter (Shareware and good cheap alternative to Photoshop ) There is also an open source (Free) image editor called GIMP this is a UNIX application and needs you to install the X11 environment on your computer. However this is a separate technical discussion!