I posted this somewhere else here, and I think it helped that guy, so maybe it'll help you too:
Well, let me get you started:
A major scale has a certain set of intervals, normally I would write them as 1 2 3 4 5 6 7, but that doesn't explain much, now does it?
Here is the pattern for the Major scale:
------------------
------------------
-----------------
------------4-6-7
------4-5-7------
--5-7------------
Here is the pattern for the Natural Minor scale:
--------------------
------------------
------------------
--------------5-7
--------5-7-8-----
--5-7-8-----------
Notice that compared to the Major Scale, the Minor Scale has a flat 3rd, 6th, and 7th degree.
In music, almost everything is compared to the major scale, so we would call the intervals of the minor scale: 1 2 b3 4 5 b6 b7.
Anyway, that's just an intro to scale theory, here's what you want to know about chords:
A major chord has these notes in it: a 1 3 and 5. So if we're in the key of C, the notes would be C, E, and G.
If we want a minor chord in the key of C, then we would have these intervals 1 b3 5. So the notes would be C, Eb, and G.
Now, about sevenths and ninths, when you are adding notes to a chord, you will need to name the notes you are adding. For example, if I want to play a Cmaj7 chord, I'm going to need to use the notes of a C major chord (C, E, and G) and then add a 7th. The seventh note in the C major scale is B, so a Cmaj7 chord would have these notes in it: C, E, G, and B.
Sometimes, when a note added to a chord is an octave above the root of the chord, musicians like to add higher numbers to the notes added. As you may have guessed, a major scale only has seven notes in it, with the root being octaved. Well, if we consider the octaved root to be "8", then a 9th would simply be the octave of a 2nd interval. That means that if you have a chord that has a "9" in it, you need to add the octaved 2nd interval of that key into your chord.
Hope that helped a little.
-Mike
