For clamshell use, you need to first connect an external keyboard and mouse (you'll need that to use it anyway). Then just hook up your external display and close the lid. Your laptop will go to sleep, but wait a couple of seconds and press a key on the external keyboard, and it'll wake up using the external display.
Remember, you also need to have the mains power supply attached.
And don't bother with anti-virus until the first Mac virus is found ... an anti-virus wouldn't do any good in that case anyway, and the Norton and McAfee offerings are crap, to put it mildly.