Originally Posted by: caaaaaseI was talking about writing a song where I play both guitars and one of them complements the other in some way, like a harmony and a melody. I'm having trouble writing the harmony...

As Jeff rightly points out, you should go through GF1 & 2 in order to get a proper basis in scales & chords. To put a fine point on it, you really need the info in the tutorials on how scales & chords are related to one another:

http://www.guitartricks.com/tutorial.php?input=426

Start there and go forward for a while.

Next, in order to understand harmony properly you should have a look at my tutorial on the basics of music theory:

http://www.guitartricks.com/tutorial.php?input=495

Then, have a look these tutorials on the basics of improvisation. They will give you an introduction to how to play a melody line or solo ideas over a set of chords:

http://www.guitartricks.com/tutorial.php?input=876
http://www.guitartricks.com/tutorial.php?input=483
http://www.guitartricks.com/tutorial.php?input=491

Hope this helps. Let us know it goes. Have fun!