This post will teach you some basic music theory and help you use that theory to key your songs. If you have a piano keyboard available, or something equivalent in software, that you can use to play along with the lesson, it will go a long way to helping you understand these otherwise very abstract concepts.

--- NOTES AND SCALES ---
Western music is almost always composed of the same twelve notes. These twelve notes comprise the "diatonic scale," and each one has a name. The easiest way to visualize the notes and their relation to each other is to look at a piano keyboard:

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The white notes are called "naturals," and each note has a name: C, D, E, F, G, A, and B.

The black notes are called "flats" or "sharps," and each has two names. The note between C and D, for example, can be called both "C sharp" and "D flat," and these are written "C#" and "Db" for short. Note that B and E have no associated sharp, while C and F have no associated flat.

If you play two notes that have the same name, but are separated by twelve half-steps, those notes are said to be an "octave" apart.

You can play the twelve notes of the chromatic scale on your keyboard and hear how the relationship between each of the twelve notes is the same. Each of the twelve notes is said to be a "half step" apart. It follows that if you play notes that are two half steps apart, such as C and D, those notes are said to be a "whole step" apart.

The twelve notes of the chromatic scale are arranged into groups of seven notes called "diatonic scales," the only two of which you'll have to know about for the vast majority of your DJ career are the "major scale" and the "minor scale."

- THE MAJOR SCALE -
The easiest major scale to learn is the C major scale, which comprises the notes C, D, E, F, G, A, B, and the C an octave above the C at the base of the scale. Since the scale starts and ends at C, C is said to be the "tonic" of the scale. If you look at the relationships between the keys, you'll note that there are whole steps between most of the notes, with two exceptions:

C
whole step
D
whole step
E
half-step
F
whole step
G
whole step
A
whole step
B
half step
C

Note how E, the third note in the scale, is two whole steps (four semitones) above C. This relationship is called a "major third"; E is thus a major third above C.

All major scales have the same relationships of whole steps and half steps between their notes. If you were to raise the tonic a half-step from C to Db and play a Db major scale, you'd play Db Eb F Gb Ab Bb C D. Notice how, as you play both scales on your keyboard, they have similar feels to them, despite having different tonics.

At this point, you should be able to select any note on your keyboard and play a major scale based on that note, using the same series of whole and half steps.

- THE MINOR SCALE -
The easiest minor scale to learn is the A minor scale, which, like the C major scale, uses only the naturals on the keyboard: A, B, C, D, E, F, G, and the A one octave above the A which starts the scale. Note how there are the same number of whole and half steps, but that the relationship between those steps is somewhat different:
A
whole step
B
half step
C
whole step
D
whole step
E
half-step
F
whole step
G
whole step
A

In this case, the third note in the scale is only three semitones above the tonic. This relationship is called a "minor third"; C is thus a minor third above A.

Like all major scales, all minor scales have the same relationships of whole and half steps between their notes. If you were to modulate up a half-step and play a Bb minor scale, you'd play Bb C Db Eb F Gb Ab Bb. Notice how, just like the major scale, as you play both scales on your keyboard, they have similar feels to them, despite having different tonics.

At this point, you should be able to select any note on your keyboard and play a minor scale based on that note, using the same series of whole and half steps.

--- KEYING SONGS ---
All songs that you'll hear are in at least one key. The vast majority are in only one key, while a few change keys (this change is called a "modulation"). For now, we won't worry about modulation, but be very aware that it's out there more than you think.

To find the key of a song, you have to listen to it carefully. All songs (or at least those that are at least somewhat interesting) work on the principle of building up and releasing tension. Most songs that you listen to will seem to "resolve" or "come to rest" on a certain note; this note is usually the tonic of the key in which the song is written. Generally, the melody of a song will tend to end on the tonic.

Be aware that finding the key is very much a holistic process, and that you can't robotically use a single technique to find the key. You have to develop a feel for it.

Let's use the melody-analysis technique to find the key of a song that you probably have in your library: "Heartless" by Kanye West (album cut).

----
In the night I (F Db Bb)
Hear 'em talk (F Db Bb)
Coldest story (F Db Eb F)
Ever told (Gb F Eb)
Somewhere far (Eb F G)
Along this road (F Eb Db Bb)
He lost his soul (Bb F Db Eb)
To a woman (Db Eb F)
So heartless (Db Eb Db)

How could you be (Db Eb F)
So heartless? (Db Eb F)

How could you be (Db Eb F)
So heartless? (Db Eb Db)
----

Note how the melody of the chorus seems to be focused on Db and F. Not coincidentally, these two notes are a major third apart.

The verses use a very simple method of building and releasing tension: the melody is almost entirely F, with repeated resolution to Db.

So, without playing another note, you can confidently guess that this song is in Db major. Note how if you just hum Db throughout the entire song, it seems to fit, and if you play a Db major scale while listening to the song, all of the notes seem to fit.

If you think the tonic might be F, you're wrong: the next note in the scale is Gb, a half-step above F, and so F can't possibly be the tonic.

Let's listen to another pop song you probably already have and figure out its key: "Lovegame" by Lady Gaga (album cut).

First, the synth riff that leads off the song provides a clue:

B B B G Gb E E E Gb D

The other big clue comes once the chorus starts. The chorus is only two notes: B and D, which, not coincidentally, are a minor third apart. Note how the entire song seems to want to come to rest on B. Thus, the song is in B minor.

To key songs where you can find the tonic, but where the third isn't obvious, play the tonic, and then play up to both the major and the minor third. One will generally sound "right," and the other will generally sound discordant or "off." Some songs have so little tonal information in them that they aren't really in a major or a minor key, but for the most part, you should be able to tell fairly quickly what the key is.

- WRITING KEY NAMES -
Major keys are noted as just the name of the tonic. So, for example, "Heartless" is in the key of Db (spoken as "D flat" or, sometimes, "D flat major").

Minor keys are noted by affixing "m" after the name of the tonic. "Lovegame" is in the key of Bm (spoken as "B minor"). If you have a minor key with a flat as the tonic, you follow the same rule; a song in D flat minor is noted as "Dbm."

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That's it for now. Let me know if you find this tutorial useful or where you need more help, and I'll clarify things as needed. I'll probably write another in a while about how to mix harmonically using the Circle of Fifths.