Rear Derailleur Adjustment
These instrctions should solve all shifting issues, however they may be overkill for most users. Most new bikes come with straight rear derailleur hangers, properly adjusted b screw, and properly set H & L screws. Hence steps #1-#3 are not required for for most new bike owners. However going through all the steps should solve shifting issues if sinply going through #4 is not sufficient.
Step #1 Make sure the rear derailleur hanger is straight
Step #2 Adjust the b screw
Step #3 Adjust the H & L screws
Step #4 Adjust the cable tension via barrel adjuster.