If you are working on a PC, you can hit the "Print Screen" button on your keyboard when you are at a screen you want to capture. Then go into Photoshop and create a new document. You should notice that by defualt, the canvas size is set to your monitor resolution. After you have created the new document, use the "Paste" function and you will have your screen shot. You may have to crop it. There are also many screen capturing programs available, WinGrab is a free one you can get off of the web.