Before you Start Tapping You have to learn the basics professionally, it will save you a ton of time and effort, I have seen so many people not able to learn advanced guitar because they haven't mastered the simple stuff first. This is the best program I have yet to see in teaching the basics to advance guitar, it also is the cheapest, Click Here, to check it out.
A Simple Tap

So finally we get to tapping, we have talked about everything else, but this is what you have been waiting for. Lets start out by attempting your first tap... I'm guessing its not most peoples first tap but bare with me. You want to start out by slamming your first finger down on the fret of your choice, lets say 1st string 5th fret. After slamming your finger down on the fret you will want to pull down to make the note ring out. You are going to have to tap pretty hard for the sound to register, and pulling off can take some getting use to, just make sure that when you pull off your not just simple letting up with your finger but actually pulling downward like your finger is plucking the string.
Tapping Exercise Number 1
First tapping exercise is a classic, the first one I learned and now it (maybe) will be the first one everybody out there learns. Start out with tapping the first note on the 12th fret 1st string, then pull off to the 5th fret and hammer on to the 8th.
It will look like this:
When doing this exercise keep your left hand, first finger positioned on the 5th fret.
Note: When tapping; the further up the neck you are tapping the worse the sound will be, because the note will ring out on the other side of your finger. To prevent this dissonant sound on higher frets keep a finger planted on a lower fret to tighten the tension on the string and keep that resonating sound to a minimum.
Tapping Exercise Number 2
This exercise is a bit harder, and takes moving the tapping finger around a bit more. Tap on the 7th and 9th use your left hand to hammer on 0 4th and 5th fret. As shown: