You have to use macro substitution in this case

lcFieldname = "firstanme"
lcSomeVar = "a"
select &lcFieldname ;
  from testdata ;
  where &lcFieldname == lcSomeVar ;
  into cursor test 

>how can I make a field that I want pick in sql statement generic. For example I can write the following sql statements
>
>select firstname from testdata into cursor test where firstname == a
>select lastname from testdata into cursor test where lastname == b
>select address from testdata into cursor test where address == c
>
>how can I make it so that the selection fields can substituted with a variable? for example
>
>select fieldname from testdata into cursor test where fieldname == somevariable
>
>I want to be able to choose my own fieldnames.
>
>Thanks
>Nick Patel