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.