It's kool, man. Everyone starts somewhere and you aint the only one asking the same questions. You can try the search option on this site for that very reason. If you have a questions about "how to use modes", you'll find everything you probably need to know.

Keys and scales.

When someone says the key of the song is C major. The song uses the scale of C major. A scale is a sequence of notes from root to root. The root is always the note before the scale name. Example C major: "C" is the root, "Major" is the type of sequence of notes. The major sequence is whole, whole, half, whole, whole, whole, half. This done determining the distance between one note in the scale to the next. The C major scale is C, D, E, F, G, A, B, C. So from the first C to the note D is a whole step. Then from D to the note E is another whole step. And it goes on finishing the sequences. So if you wanted to write a song in the key of C major, you would use the C major scale.

Memorizing keys and scales is easier to remember if you learn there patterns on the fretboard. Look up scales on in the tricks part of the site or try yahoo "guitar scales" and you'll get a list of them.

Ok when it comes to chords within a key. They are all based on triads which are formed by putting together the root the 3rd and the fifth. So a C major chord is C, E, G. Check the C major scale above. It's C, skip a note, E, skip a note, G. If you do the same with every note within the C major scale, you end up with.

C major (CEG)
D minor (DFA)
E minor (EGB)
F major (FAC)
G major (GBD)
A minor (ACE)
B diminished (BDF)

That's how you come up with chords in a key or scale. To write a chord progression in C major, you would use a sequence of chords listed above like ||Cmaj |Gmaj |Amin |Fmaj ||.

Learning the notes on the fretboard.

Here's a program to help learn the notes on the fretboard.
http://www.francoisbrisson.com/fretboardwarrior/download.html

Modes.

A mode is a scale but instead of starting on the root as meantioned before, you start on another note within the scale. Take the C major scale again, C, D, E, F, G, A, B, C. If you started with D instead of C you would end up with a mode, D, E, F, G, A, B, C, D = D dorian mode. you can do this with all 7 notes, each is a different mode. Starting with A produces the A minor scale: A, B, C, D, E, F, G, A.

List of modes from C major scale:
Major scale (Ionian) CDEFGABC
Dorian mode DEFGABC
Phyrgian mode EFGABCDE (used alot in metal)
lydian mode FGABCDEF
mixolydian mode GABCDEFG
Minor scale (Aeolian) ABCDEFGA
locrian mode BCDEFGAB (very dark sounding)

Chords

Learn open position chords first. Like C major, D major and minor, G major, A minor and so on. Look up open position chords and you find a bunch of them. Out of the thousands of chords, there is a smaller collection of common ones for the guitar; start there.

Hopefully I helped you out man.