The job of practicing scales on the guitar can be quite tedious. It is hard work without any immediate reward and the long term reward is kind of not very clear. There is a reason why you need to learn scales on the guitar. When you are improvising or composing guitar solos your fingers need to be able to go from one note to the next without your brain having to think about it. The end result when you learn to play scales is your mind can relax and play with musical ideas instead of needing to think, "where do my fingers go next?"
There are two ways to practice guitar scales. The first is to play as fast as you can with a maximum amount of tension and a minimum of precision. This kind of scale practice is characterized by fluffed notes and a distinct lack of improvement. The other way is to play your scales slowly and carefully, trying to keep your arms relaxed as you play. This way you are always learning something about how your body works and what you can do to improve the way your scales flow as you practice.
The major scale is a good place to start. This scale is a series of notes with a definite number of intervals or steps between each note. For a guitarist a step is one fret. The distance between D and E is one step and the distance between F sharp and G is a half step, which means G is the very next fret up from F sharp. If you are playing in the key of D, the notes of the major scale will be: D E F# G A B C# and D.
Another scale you will be seeing mentioned in guitar literature is the pentatonic scale. This scale contains five notes. The notes of the pentatonic scale are the first, second, third, fifth and sixth notes of the major scale. So the in the key of D the notes are D E F# A and B.
If you become familiar with the pentatonic scales in a few keys, you will be able to compose guitar solos using the five notes of your pentatonic scale played at various locations on the fretboard. Then it will be your turn.