On a PC: Press the “Alt” and “Print Screen” keys simultaneously while the window you want a picture of is active. [There really is a key called “Print Screen” near the top right of most PC keyboards!]. You now have the image in your clipboard; paste it into a blank email and send it to your instructor.
On a Mac: Press ⌘-Shift-4 and then drag the cursor to select the browser screen; when you let go, you’ll hear a click. You’ll now find a document on your desktop whose name begins with “Screen Shot”; email this to your instructor as an attachment.
On a Mac or PC, using the Firefox browser: Look for and install the browser add-on “pdfit” (it just takes a second). Once it’s installed, go to the Tools menu in Firefox, click on “Pdf It!” and then select “Save as PDF ->Visible Area”. Choose a name for the pdf (including the .pdf extension if that doesn’t automatically happen) and email it to your instructor.
If none of the above options work for you, try searching your computer’s Help menu for something like “Screen Shot” or “Take a picture of what is on your screen”!