Understanding the major scale will open up the guitar for you in a way that nothing else can! When you have learned the notes that make up each scale and the chords that are made from those notes, you will not only have essential tools for working with countless songs in major keys, you will also be able to easily learn all of the other scales and modes that you need. In addition, you will be able to communicate more effectively with other musicians and navigate keys and tonal centers in your improvisations.
Begin by learning the formula for the major scale:
whole step, whole step, half step, whole step, whole step, whole step, half step
Next, construct all 12 of the scales. You can check your work by referring to the chart below.
Notice that a correctly spelled scale will have one note from each letter name and will not mix up sharps and flats!