What is a mode?
A mode is a formation of note steps. This is one fret on your fingerboard. Each fret is a half step. Therefore, you can assume that all e's are a half step from f, and b's a half step from c, respectively. So if you are in the key of C you will end up making a certain number of whole and half steps to reach each solid note ( D, E, F, etc.).
The pattern for which these steps make a scale is called a mode. You can make these modes with any given note. You simply count the whole and half steps to each note. A whole step being from one note to another, excluting B to C, and E to F, which are half steps, as described earlier. So in general, starting at C you can go to D in a whole step, then from D to E in a whole step, From E to F in a half Step, F to G in a whole step, G to A in a whole step, A to B in a whole step, and B to C in a half step. That pattern of steps is the mode for C, or the Ioninan Mode. It looks like this for easy reference:
WWHWWWH = Ioninan
So given this information, you can assume that from D, all you have to do is remove the first whole step and place it at the end. Like this...
See? The first whole step which represented C to D is now at the end because we are starting on D so that step goes at the end to maintain the seven steps in an eight note scale. These scales can be done with more than eight notes, but they will just repeat themselves, so I will leave that to you.
Here are all of the Modes and their titles for easy reference:
WWHWWWH = Ioninan
WHWWWHW = Dorian
HWWWHWW = Phrygian ( Pronounce; Fridge-E-an)
WWWHWWH = Lydian (Pronounce LID-E-an)
WWHWWHW = Mixolydian (Pronounce MIX-O-LID-E-AN)
WHWWHWW = Aeolian (Pronounce AYE-O-LEE-AN)
(this final one is rarely used, but is for the key of B)
HWWHWWW = Locrian
These Patterns start Ioninan in the Key of C, Dorian in the Key of D, and so on. Also, the third fret on your second string is a middle C, so if you want to attempt to construct these scales you can use that reference.
The thing with rock, is that they are not always playing in the same key. Someone can play the bass part in a G, but if you are using the Mixolydian scale in any key, it will sound cool. Learning to use this to your advantage will help you bust out those facemelting solos by your favorite artists. You need to use the Key that corresponds with the piece, like the minor form if you want to do that. Otherwise you can play the same key with a different mode. Such as a G with a Locrian mode, which would be the minor mode, or the Phrygian mode, which doesn't have much connection, so it will create dissonant sounds that are more of the heavy metal genera. I hope this lesson was of value to you, and have fun practicing, make sure you do it right...