Hey... I was wondering if anyone could give me a good practical explaination of the greek modes. I would really like to know how to use it in writing solos... Are there any rules?

I am guessing you are talking about the scales usually referred to as "modes". They all have names related to ancient Greece, such as "Lydian" and "Aeolian". If that is correct, just call them 'modes', not 'greek modes', to avoid confusion.

Next, modes are just scales, like any other. In fact, the Ionian mode is just another name for the standard, 7 note major scale. The Aeolian is the same as the standard, 7 note Natural Minor scale. Use them in the traditional manners.

I think the most basic way that scales are used by guitarists is to provide the basis for making a melody line (a lead) over a harmony (say a rhythm guitar playing chords). Alternately a songwriter or composer could harmonize a melody line they like.

Hmmm... this is kind of a big subject (though not rocket science). Do you know what a I-IV-V or a ii-V-I chord progression means? Do you know how to harmonize a scale? This can help me answer your question.

Basically, harmonize (make chords appropriate to the mode for each of the notes in that mode) for each of the modes, then make any chord progression you like out of those chords and create the melody over that progression, using notes from that same mode.

Learn to recognize these little progressions in songs, or add them to songs you write. Or base entire songs around them.

If that last stuff didn't make sense, try this:

To give you something to do/play with/think about, try playing an E chord (any rhythm you like) for a few beats (say, 8), then do the same with a D chord, then returning to E and then D again and so on. Think of this as a I - bVII progression in E. You can play a lead over this progression using the notes from the E Mixolydian mode (e, f#, g#, a, b, c#, d).

Another one to try would be to play notes from the A Dorian mode over an A minor7 - D7 A min7 - Emin7 (then repeat the whole thing starting back at Amin7) chord progression. Think of it as a i7-IV7-i7-v7 progression and just noodle around in A Dorian (a, b, c, d, e, f#, and g) over it (you could record the chord progression, or get a friend to play the chords) to get the feel for the Dorian mode.

There's more you can do with the modes of course, but excercises/jamming like this will help you get a feel for the possibilities. You might want to tackle the modes one or two at a time, to really get the sound and fingering down. I'd start with the 7 note major and natural minor scales (i.e. Ionian and Aeolian) if you don't have them down already, then Dorian and/or Mixolydian (both are quite common) and then the remaining three.

Hope I answered the right question, and hope this helps!

DMT gives good advice, but I'd like to add that you should learn the modes simply to have a better understanding of the major and natural minor scale, as each mode is basically a different position, on the guitar neck, of these two scales.
Furthermore, examine each mode and try to understand the similarities between them. Notice that ionian, lydian, and mixolydian all have a major 3rd. Furthermore, the only difference between mixolydian and ionian, is that the mixolydian has a flat VII. When soloing over a chord progression based on the mixolydian, it is nice to throw in the VII of the ionian as it is a better resolution to the root.
Notice also that the locrian and phrygian both have a flat II which gives a diminished, almost forboding quality to these scales. These are the hardest to master, but can yeild very original, albiet dark melodies.
Become familiar with the similarities of each mode, and try exchanging two that are similar. This is especially effective in mid-riff, and can add quite a bit of dimension and originality to your melodies.