Many novice players ask for advice about which keys of harmonicas they should start out with.
To determine which keys will be best for you to buy, ask yourself these two questions:

1) Will I be playing solo, or with others?
2) What kind of music do I want to play?

The following information relates only to the Major Diatonic tuning.

WILL YOU BE PLAYING SOLO, OR WITH OTHERS?

If you intend to play solo, or simply for your own satisfaction, then any key can be used. In other words, any song can be played in any key, regardless of what key the original composer, or artist, may have selected.

However, if you plan to play with someone else, such as a duo or group, or to play along with a favorite recording, your key selection would be governed by those situations.

WHAT KIND OF MUSIC DO YOU WANT TO PLAY?

In order to know which keys of harmonicas to buy, it is necessary to first determine which style of playing you will be using, Straight Harp (1st position) or Cross Harp (2nd position).

If you want to play simple melodies and folk music, you will probably be playing in Straight Harp. This means you would start from the blow, accenting the blow notes and blow chords. You would use a C harp to play in C.

If you want more "expression" for playing Blues, Rock, Country and Pop music, you will probably be playing in Cross Harp.

This means you would start from the draw, accenting draw notes and draw chords, and bending. You would use a C harp to play in G.

90% of today's players use the Cross Harp style for Blues, Rock, Country and Pop music.

Note:

It's important to understand that although the notation layout for each key of Major Diatonic will be different, the PATTERN of notes, chords and bends will be the same.
This means that once you learn to play "Jingle Bells" on a C Major harp, you can play "Jingle Bells" on any key of Major harp (G, A, Bb, Low F, etc.) using the exact same blow/draw patterns.

The biggest selling keys of Major Diatonics are:
C  A  G  D  E  F  Bb   Eb  - in that order.

If your goal is to be able to sit in with others, jam with bands and play in most any situation, it would be ideal to have all 12 keys, plus Low F and High G.

However, if you are just getting started, owning the first 5 keys listed above would give you enough flexibility to play in many situations.