Are you trying to run this off the computer's power supply?
In a standard supply the the 12V lines on a four pin molex will be yellow, that plus the black ground wire is all you need to get 12V. You'll want to make sure you have the excess capacity to run this LED strip (though most likely, doesn't look too drastic)
The power supply brick should indicate which is positive and negative, the center or the outside. Simply wire your pigtail up to match and it should fire right up.
Image below indicating that the center post is positive:
http://itp.nyu.edu/archive/physcomp-spring2014/uploads/power_supply_02.jpg