When you are at the screen you want to take a screenshot of press the key PrtScn on your keyboard
(normaly located at the top after F12 and above Insert).
Load up Paint (or any other graphics program), goto file -> paste.
Your image should appear in the window, edit image if you want, save image as a jpg.
Goto
http://imageshack.us/ click browse locate your file on your hard drive, click ok, click host it.
Wait for your image to upload then paste in the link from the last box 'Direct link to your image' into your post.