Finally got home so I can potentially give some intro that can help. Google/wikipedia/youtube is going to be your friend if youre really starting out with basics or if something here doesnt make sense.
Let's use C major in the example.
Before we can even start, what's C?C is a note which is a certain tone. There are 13 notes between C of one octave and C of a different octave (C, C#/Db, D, D#/Eb, E, F, F#/Fb, G, G#/Ab, A, A#/Bb)
# = sharp
b = flat
Mess around with the piano role in your DAW of choice. Frequency of sound is what gives each of these notes the perception of a different pitch.
What is major?Good question. Often times major is referred to as "the happy sounding scale" vs minor "the sad sounding scale".
Major (Ionian mode) is comprised of certain intervals between each notes of the scale (whole-steps/half-steps). Using the C major scale, our major scale is C D E F G A B C. Play it on the piano/in your DAW and you'll recognize it. The pattern here is C -whole step- D -whole step-E-halfstep-F-wholestep-G-wholestop-A-wholestep-B-halfstep-C. Imagine wholes step meaning we had to take 2 "steps" on the piano to get to the next note (to get from C to D we had to pass over C#, but for E to F, we didnt have to pass any other notes).
You can apply this pattern when starting the scale of any note which means you've now learned all the major scales.
This is great so we can finally start writing chords (in an Ionian mode). Now for a song to be in key, we can only use the notes in the scale (unless we're using borrowed chords but let's not get into that now).
For major, if we're working with triads (a group of 3 notes) for a major mode, we can apply the following:
major, minor, minor, major, major, minor, diminished
Sounds like this
https://clyp.it/2fraj2u2.
For C major, this ends up being Cmaj(I),dmin(ii),emin(iii), Fmaj (IV), Gmaj(V), amin(vi), bdim(vii
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Now we're in business. Start rearranging these and you're writing music. Common progressions are I IV V vi, or I V vi IV
https://en.wikipedia.org/wiki/I%E2%80%93V%E2%80%93vi%E2%80%93IV_progressionHopefully I didn't jump into too much too quickly. To be honest, this is something that'll take some time to learn, longer to completely understand, and years to apply to your own music.
If anyone has any questions, feel free to ask me.