Every musician has to know it, so I'm here to help you out.
First off, it is most important to know about the key. If you don't know the key, you can't play along or solo. 90% of the time, the key is the first note/chord the song is. If the opening chord is C, than the song is in the Key of C.

C is the same as Am.

W stands for Whole Step from the previous note.
H stands for Half Step from the previous note.
M stands for major.
m stands for minor.

When you're forming the chord structure for songs, it is important to know this:
C Dm Em F G Am Baug/dim C
Do Re Mi Fa So La Ti Do.
1 2 3 4 5 6 7 1 or 8
W W H W W W H

The basic modes:
Ionian. In the Key of C, this is the C. It is major.
Dorian. in the KoC, this is the Dm. It is minor.
Phrygian. In the KoC, this is Em. It is also minor.
Lydian. F. Major.
Mixolydian. G. Major.
Aeolian. Am. Minor.
Locrian. Bdim. Diminished.

There are others, but these are the basic.

Basic chord forms.
Chords based off the A:
e-0
B-2
G-2
D-2
A-0
E

To move to a B, simply slide up 2 frets. By barring the second fret.

Chords based off the E:
e-0
B-0
G-1
D-2
A-2
E-0

Once again, to move to F, slide up 1 fret and barre the first.

Minor chords are major chords with a lowered third.
A becomes Am:
B-1
G-2
D-2
A-0

This is also a moveable chord.

E becomes Em.

G-0
D-2
A-2
E-0

Moveable. Slide up one for Fm.

Examples of the modes in music. (Thanks Gear)
Mixolydian: Paul Gilbert - "You Kids".
Lydian: Rush - "Freewill".
Dorian: Steppenwolf - "Born To Be Wild".
Aeolian: Led Zeppelin - "Stairway to Heaven".
Phrygian: Alice In Chains - "Would?".
Locrian: Judas Priest - "Painkiller

... i still dont get it

What's not to get?