How to hail black cabs in London?

It may sound a bit weird to ask, but I’ve never in my life hailed a cab. As a small town girl, I’ve never had the opportunity. But I’ll be leaving in the morning for London, England for a semester abroad, and I have to take a cab halfway to the orientation center. What do I do to hail one down? And how can I tell which are legitimate black cabs and not the unlicensed ones?