Probably the most embarassing thing for me, is that when I first started learning guitar, I hated chords and skipped past them.
Straight to any songs.

I have a very small knowledge of music theory or chords, I just play with tabs.

I know only a few chords..

I must learn music theory/chords for songwriting.

Is there any tutorials or lessons?

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There's a bunch of articles on music theory here. In fact, there's a whole section of columns about them: http://www.ultimate-guitar.com/columns/music_theory/

and if you just want to learn chords, go to like http://www.all-guitar-chords.com/ or go out and buy a chord book. Doesn't really matter which one (IMO), just find one that you like.
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Here's an excellent guide that covers plenty on chords. You can just skip around there for the lessons about chords, you don't need to run through them all.


Also, these 2 videos (and the pdf's that come with them) from Andrew Wasson, an admirably accomplished guitarist, should help loads. I recommend looking through all of his videos, actually.

I think the easiest way to use chords - if you really break it down into the simplest way of looking at it - is that you can extend your Em, Emaj and Am, Amaj chords into barre chords, and play them anywhere on the neck.

If you want to get slightly more technical, you can build a major or minor triad from a root, a 3rd, and a 5th of any scale. If you wanted to play a G major triad in the key of G major, just find the notes for the 1, 3, and 5 in that key, and you've got yourself a major triad!

In the case that you don't know what series of whole and half-steps makes up a major or minor chord, just know this:

Root to 3rd = 2 whole steps
3rd to the 5th = 1.5 steps. (or three half-steps, depending on how you look at it)

Root to 3rd is 1.5 steps
3rd to the 5th is 2 while steps.
The minor chord is basically a major chord flipped upside-down.

Rroot to the 3rd = 1.5
3rd to the 5th = 1.5

If you don't know anything about the different qualities of the chords that correspond with the different degrees of scales, it goes something like this:

I - ii - iii - IV - V - vi - viio

...With the upper-case roman numerals being major and the lower-case ones being minor. The "viio" means that the chord is diminished, and the "o" would usually be replaced by a symbol that looks like the Celsius or Fahrenheit degrees circle. But it's late at night and I don't feel like figuring out how to type that symbol.

Seventh Chords:
Seventh chords are like triads, but include the 7th degree in addition to the 1, 3, and 5. Like with major or minor triads, a 7th also has its own quality: depending on the distance from the root, the 7th can be major or minor as well. The easy way to figure out a 7th chord's quality is to count downward from the root. A 7th that is a half-step down from the root is major while a 7th that is a whole step away is minor.

In this way, we can have Major-minor seventh chords, as well as Minor-major ones.
They'd look something like this:
GmM7 or Gmajm7
Don your antithesis spectacle, your backwards monocle, and traipse forward to victory.
Well, I just spend a half-hour typing up that lesson, only to find that everybody else has answered the easy way with links. At least I proved that I know something.
Don your antithesis spectacle, your backwards monocle, and traipse forward to victory.
Yo man you wanna take jabs? How about.. answered the smart way, instead of spending "a half-hour typing"?

Oh and if that wasn't a little jab.. I'm just being a stinky homo, forgive me?
Haha, yeah. Thanks, I'll keep that in mind.
Don your antithesis spectacle, your backwards monocle, and traipse forward to victory.