Taking a screen shot is pretty easy. First, just find the "PrintScreen" button on your keyboard. Pushing that takes a screen shot and puts it into your clipboard. Now you'll need to go into Paint or some other image application. Then just paste it in with a "Ctrl + V" or Edit->Paste, etc.

From there, you save the picture as a .jpg or .gif file and you can upload it into your post by clicking the image upload button.