Do you want the whole screen?
Command-Shift-3: Take a screenshot of the screen, and save it as a file on the desktop

Do you want just the window you are working in?
Command-Shift-4, then space, then click a window: Take a screenshot of a window and save it as a file on the desktop

Do you want to select a particular part of the screen?
Command-shift-4  (your cursor will become a cross you can drag across and down to select the area to make a screenshot)

Now if you want to take the screenshot and just save it to the clipboard you will do the same thing only add ctrl.  For example: Command-Control-Shift-3
The same with the others – just hold down ctrl too.  Why would you want to do this?  Instead of having this picture sitting on your desktop you might want to open your email and paste it there and send it to me to show me a problem you are having.  You might want to paste the picture into a document or a presentation to show a step in a process you are teaching.

If you just got your Mac, you’re operating system version is Leopard and you can also do the following:
Hold down space to lock the size of a selection and move it when the mouse moves.
Hold down shift to resize only one edge of the section
Hold down option to resize selection without the center moving.
(Because I am using Tiger I haven’t had the opportunity to try those last keystrokes out but I have used the other commands often.  If I have made an error with these last commands I hope someone will let me know!)