There are several reasons why your guitar sounds bad, but the most common one is the tuning of your guitar in the higher positions of guitar's fretboard. So, if you play the E string, fretting the 12th position should return E, an octave higher. To figure out if your guitar matches this criteria, tune the string in DGT and then play the 12th fret on the same string. If it moves away from the center, you should make some adjustements on your guitar.
Solution is different by electric and acoustic guitars.
Adjusting these screws changes the tuning of your guitar. Check the status of the 12th fret. If the result is unacceptable, turn the proper screw with screwdriver. After turning the screw and retuning, if the ray appears to be more near the center, you are turning the screw in the proper direction. With this screw you can adjust the tension of guitar's neck.