Access table field on creating a trigger postgresql
Per documentation:


  The trigger function must be defined before the trigger itself can be
  created. The trigger function must be declared as a function taking no
  arguments and returning type trigger. (The trigger function receives
  its input through a specially-passed TriggerData structure, not in the
  form of ordinary function arguments.)


Bold emphasis mine.

To pass those parameters:


arguments

  
  An optional comma-separated list of arguments to be provided to the
  function when the trigger is executed. The arguments are literal
  string constants. Simple names and numeric constants can be written
  here, too, but they will all be converted to strings. Please check the
  description of the implementation language of the trigger function to
  find out how these arguments can be accessed within the function; it
  might be different from normal function arguments.


And finally, in the chapter Trigger Procedures:


  Note that the function must be declared with no arguments even if it
  expects to receive arguments specified in CREATE TRIGGER — trigger
  arguments are passed via TG_ARGV, as described below.
  
  ...

TG_ARGV[]

  
  Data type array of text; the arguments from the CREATE TRIGGER statement. The index counts from 0. Invalid indexes (less than 0 or greater than or equal to tg_nargs) result in a null value.


The final difficulty is that SQL only works with static identifiers. If you want to parametrize a column name, you need dynamic SQL. In a plpgsql function, use EXECUTE for that.

Example

CREATE OR REPLACE FUNCTION trg_foo()
  RETURNS trigger AS
$func$
DECLARE
   _result text;
BEGIN
   CASE TG_OP
   WHEN 'INSERT', 'UPDATE' THEN
      EXECUTE format('SELECT %I FROM %s WHERE tbl_id = $1', TG_ARGV[1]
                                                          , TG_ARGV[0]::regclass)
      INTO  _result
      USING NEW.sometbl_id;
   WHEN ...

   END CASE;

   -- do something with _result ...

   RETURN NULL;   -- since it's an AFTER trigger
END
$func$  LANGUAGE plpgsql;


CREATE TRIGGER foo_trigger
AFTER INSERT OR UPDATE OR DELETE ON foo
FOR EACH ROW
EXECUTE PROCEDURE trg_foo('foo', 'field'); -- two params: table, field


Answer on SO with details about the cast to regclass:
Table name as a PostgreSQL function parameter