If then else

Hi I am not very good at coding.  

I have a table call invoice item and in it  there a choice dropdown field called 'Service Description' ,  if I choose the dropdown field of initial acupuncture , I want the field Item No  automatic show number 103:  my coding is not working so far, could you help me please? thanks

If 'Service Description' = "Initial Acupuncture"

then 'Item No'=103