How to Hold A Diatonic Harmonica
Most diatonic harmonicas have the numbers 1 to 10 stamped on the top cover, right above the holes - that side is the top. It IS possible to play the harmonica upside-down, and some people get used to playing them that way, but it is not recommended that you do this because then you won't have any numbers to work with.
The whole numbers thing is hard enough to work with as it is, anyway - that's why the Harmonica SuperStart system has a SuperStart Card that goes on your harmonica so you can see where you are on the harmonica all the time - otherwise it is kind of like this for the beginner:
How to Hold A Diatonic Harmonica
The most important thing is to have the numbers on top. If you look closely, you'll see these tiny numbers, one above each hole. These are the things that you reference to when you play your typical harmonica tablature song.
Have you ever pretended that you were "pinching" like a crab? Most likely the fingers that you pretend to pinch with are your thumb and forefinger.
Go ahead and "pinch" a little bit with either hand, and then open your "pinchers" (thumb and forefinger) wide and insert your harmonica into "the pinch" as far as it will go - you should now be holding or pinching your harmonica with just your thumb and forefinger.
If the "pinch" analogy doesn't make sense to you, then try looking at it this way:
If you hold up just the thumb and forefinger of each hand, then your left hand will make the letter "L" shape, and your right hand will make a backwards "letter L" shape. Put your harmonica where the top of the L joins the bottom of the L (the juncture of your thumb and forefinger) and then close up your L.
You are now holding the harmonica with just your thumb and forefinger - this is useful because, if you notice, that your three fingers still free on that hand are able to wiggle all they want.
A neat little experiment you can do with those three fingers is just to flap them up and down quickly while blowing and drawing on your harmonica - did you notice a "wavery" sound when you did this?
That "wavery" sound is called vibrato, and by flapping or wiggling your middle, ring, and pinkie fingers while blowing and drawing your harmonica held in between your thumb and middle finger, you are creating vibrato, which can be an enhancement to your harmonica sound (depending on how you use it) or it can be an effect that you choose to use or leave out of your playing.
A big reason that I'm first introducing you to holding the harmonica in just your thumb and forefinger is this:
A lot of harmonica players get used to creating a "cup" with their hands while playing the harmonica, and play that way almost exclusively. A lot of books (most of them) have you start off creating this "cup" with your hands and want you to get used to playing this way.
The trouble with playing with the harmonica cupped in your hands is threefold:
1. The cup cuts down on your volume considerably - and believe me, you'll run into lots of situations where you'll want to produce all the volume you can, to be heard over the other instruments.
2. The cup technique gives you a false sense of the "fullness" of your harmonica tone - with the harmonica cupped in your hands the sound seems bigger and richer to you, and you can feel the air rushing in and out inside that cup as you play - it just plain seems bigger and richer. The problem is, that other people won't experience your harmonica sound that way - to the greatest extent, they experience the sound that exits your hands, not what you are experiencing inside your hands. The more you can work with a "naked" tone, using an uncupped technique, the more you will be hearing and working on your own actual tone that people will hear and will be recorded.
3. A cupped harmonica is not very good at all in front of a microphone - the hands cut the volume way down and also make the harmonica sound muted and muddy. If you ever encounter a situation where you are suddenly asked to play in front of a microphone (and I hope you do!) then you will be glad that you are used to playing with an hands-open technique instead of cupped hands. When your hands are open, all you have to concern yourself with is how far you are away from the microphone (and usually its good to not get too close, unless the band's soundman is actually paying attention - a rare occurrence!) instead of wondering the whole time why no one is hearing you with your hands closed into a cup half of the time.
If you ever find yourself in a recording studio, this holds true even more - the excellent studio microphones will pick up the excellent tone that you have learned to create with your open hands, instead of the muddy irreproduceable sound of the "cup".
All that being said...
Most diatonic harmonicas have the numbers 1 to 10 stamped into the top cover, so that side is the top. When you play the harmonica, the lower notes should be on the left, so that as you play from left to right the notes play from low to high. You should hold the harmonica in your left hand, with the end of it pushed against the “web” between your thumb and forefinger. The thumb and forefinger grip the harmonica lying along the bottom and top respectively, as near to the rear of the harp as possible.
The remaining fingers on the left hand should be held tightly together to form the top of your “cup." The “cup” is the airtight “sound chamber” you will be trying to create behind the harmonica with your hands. Use your right hand to form the bottom of this sound chamber. Practice making the “cup” as air tight as possible.
When it is airtight you can feel the pressure of your blowing and drawing against your hands. When you get a good airtight cup, you can open and close it with your right hand while playing, and get that well known “wah - wah” effect.
When you play the harmonica, the lower notes should be on the left, so that as you play from left to right the notes play from low to high. You should hold the harmonica in your left hand, with the end of it pushed against the “web” between your thumb and forefinger. The thumb and forefinger grip the harmonica lying along the bottom and top respectively, as near to the rear of the harp as possible.
The remaining fingers on the left hand should be held tightly together to form the top of your “cup.

