I guess you mean tapping as in hitting the string. What you do is hit the string with your index finger (the one of your rigth hand) to make it sound, in any fret you want.
Now, when I said tapped, though, I didn't mean that. I should have said muted. I meant as that the string has the finger lying on it, therefor it doesn't sound when hit. Umm, wait, I think I'm confuseing myself with terminology :P