Just in case the previous post isn't clear to you, the word you're looking for, somewhere on your screen, is probably 'Greyscale'.
If you simply want to print the picture in B&W (rather than saving it in this form), go to File > Print. (Don't just click on the 'Print' button as this usually bypasses the advanced print settings). What you see will depend upon the type of printer you're using but it's usually necessary to click on 'Properties'. Then select 'Greyscale printing'.
If you actually want to save the picture as a B&W file, most image viewing and manipulation programs offer 'Convert to Greyscale'. This is usually accessed from either the 'Edit' menu or the 'Image' one. Once you've changed the picture to B&W, select 'Save As' from the file menu and choose a new file name (so that you don't overwrite the original colour file).
Chris