I've a stored procedure which takes field name in one parameter. How do I execute the query & store the result in variable by avoiding temporary table?

-Stored proc does not work
CREATE PROCEDURE g_GetResult(@FldNam varchar(20), @Code char(3))
BEGIN
  DECLARE @FldVal varchar(15)
  --Query will always return one row
  SELECT @FldVal = @FldNam
    FROM MyTable
    WHERE PrimaryCode = @Code

  --I'm doing a lot of stuff here with @FldVal
  ...
END

If in the above stored proc, I use the field name, it works.
SELECT @FldVal = MyFldNam
  FROM MyTable
  WHERE PrimaryCode = @Code
Where "MyFldNam" is the field name in "MyTable"

But if the field name is in variable. It assigns the @FldVal with the value of @FldNam
SELECT @FldVal = @FldNam
  FROM MyTable
  WHERE PrimaryCode = @Code

How can I use Exec with result returning to variable?