Below are the 5 most common major keys that guitar is played in. That does not mean that you should neglect the other keys, just make sure you know these first. It is important to play scales in the full range of a position. Even though a C major scale would technically only go from C-C, if you just practice that limited range you would be leaving out the notes in the upper and lower range of the first position. This is why these scales are written the way they are. You will start on the root of the scale and descend to the lowest note in the position, ascend to the highest and return to the lowest root note. As well as playing these as written, also try holding each root note ( letter name C G D A E of the key ) of the scale for twice as long. This will help establish in your ear how all of the other note relate to the root. Think about the names of the notes as you play these scales.