Depends on what you want. If you want the whole screen, hit the Prt Sc button. This puts the image in the clipboard. You can then paste the image into where ever you want it, pressing Ctrl V (Ctrl and hold it, then press V, release both)(many applications will also have a menu option Edit/Paste which does the same thing).
If you just want one window, make sure the window is the active window, then press Alt Prt Sc (as before, press and hold Alt, press Prt Sc, release). Now the clipboard contains the image of just the one window.
To have a harmeless practice, save an image, then open up WordPad, that is Start/Programs/Accessories/WordPad and paste the image into that program's work area. And there you see your image. Exit WordPad. Don't save the image as a file unless you really want to. Most applications, including Microsoft Word can read the files WordPad creates, but it is a lot faster to load than Word.
Once you have saved an image as a file, you can attach it to e-mails etc. | 
how to take screenshot 
