I reckon the important thing to remember with tapping is that you're not just tapping down on the string; it's a pulloff as well.
Also, decide which you find easier; the Van Halen method with the index finger (holding the pick between the palm and the second finger) or the more steve vai method, using the second finger to tap. Personally I find both work well in different situations.
And sometimes you accidentally hit open strings; it helps to dampen the strings with the back of your right hand (only really works with the VH method; too bloody hard otherwise

)
Or you could be a nutter and try using all your fingers.
Youtube is your friend.