Steam/boil it for 12 minutes. The sooner you can get it cooked after cutting it from the stalk the better....
12 MINUTES?! 
Geezus, you might as well buy canned or frozen if you're going to boil or steam it that long. 
3 minutes in boiling water, tops, then onto the plate and eat immediately. If the ears are a day (or more) old, you can add some sugar to the water. But never, ever, add salt, it makes the skin of the kernels tough.
Sorry...I've tried as little as 6 minutes, and the result was WAY undercooked for my taste. And as a response to your comment, I'll say that if the ears a day-old (or more), you may as well buy canned or frozen...the sugar in corn turns to starch incredibly fast; imho, if it's not in the cooker within a couple of hours of being cut from the stalk, the sweetness drops WAY off...!
3 minutes? You may was well not even cook it...
(fwiw, my only experience with this is with NJ sweet corn...)
