Understanding the major scale is a necessity if you want to understand music at all. It comes pretty close to being the foundation of all Western music. The minor scale is based on it, as are the pentatonic scales and all the other modal scales you'll eventually learn.
The notes in the G major scale are: G, A, B, C, D, E, F#, G. Here's what it looks like:
E ------------------------------------------
B ------------------------------------------
G -----------------------------0-----------
D ------------------0--2--4---------------
A -------0--2--3--------------------------
E ---3-------------------------------------
Now let's extend it up the neck:
E -----------------------------------------------0--2--3--
B ------------------------------------0--1--3-------------
G -----------------------------0--2------------------------
D ------------------0--2--4-------------------------------
A -------0--2--3------------------------------------------
E ---3-----------------------------------------------------
There are a couple of major scale patterns you'll want to learn. Notice that these are all the exact same notes; the difference is in where each note is played and the fingerings you use:
E ----------------------------------------------------2--3-----
B --------------------------------------------3--5-------------
G --------------------------------2--4--5---------------------
D ---------------------2--4--5---------------------------------
A ----------2--3--5--------------------------------------------
E ---3--5------------------------------------------------------
2 4 1 2 4 1 3 4 1 3 4 2 4 1 2
And:
E ----------------------------------------------------------3-----
B -----------------------------------------------3--5--7---------
G ------------------------------------4--5--7--------------------
D -------------------------4--5--7-------------------------------
A --------------3--5--7------------------------------------------
E ---3--5--7-----------------------------------------------------
1 2 4 1 2 4 1 2 4 1 2 4 1 2 4 1
That last pattern can be pretty difficult at first because you really have to stretch your fret hand. If you feel pain or fatigue in your left wrist, stop for a while. Remember to keep your wrist rotated forward, and to focus on slow, steady, accurate alternate picking. Also, whenever you learn guitar scales, pay close attention to the suggested fingerings to develop good habits. Keep practicing each of these patterns until it sounds smooth and steady. Now try moving the patterns up the neck. Just like the pentatonic patterns, you can move to different keys simply by moving these patterns to different root (starting) notes.
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