Just press the 'Print Screen' button on your keyboard (usually the top left button in the group above the cursor keys) and it will copy the screen to your clip board from where you can print it / save it etc etc
Atl + Print Screen captures the screen, but closes any open menus, drop down lists, etc. Ctrl + Print Screen captures the open menus, drop down lists, etc.
When you use Alt + Print screen, etc, it puts a copy of the screen in the clip board. To save a copy of the screenshot open another application (like Word) then use the Paste command (or Ctrl + V) to put the screenshot into the document.
If you want to do a bit of editing open a program like Microsoft Paint (comes installed with Windows) and you can paste the image into that. Then you can crop the picture, annotate it with text, etc before saving it as a jpg file.
... bit late, but there is also a handy free application "MWSnap" which can be downloaded HERE - select the area of the desktop you wish to copy (by left clicking and dragging the cursor) and it can be saved to .jpg, .tif, .bmp, .gif or .png - resolution (1/4/8/15/16/24 or 32 bit) can also be selected.