These instructions should help solve shifting issues. Most new bikes come with straight rear derailleur hangers, a properly adjusted b screw, and properly set H & L screws. Hence, new bike owners only need to examine Step #4, however we include each step in order for completeness.
 

Step #1 Make sure the rear derailleur hanger is straight

derailleur hanger removed

Step #2 Adjust the b screw

 b screw adjustment on bicycle rear derailleur

Step #3 Adjust the H & L screws

 h screw out

Step #4 Adjust the cable tension via barrel adjuster 

barrel adjuster on bicycle shifter