In my table Team which contains:
create or replace
AFTER insert ON Team
FOR EACH ROW
select sum(Wins*3 + Draws + Losses*0) into tmp from Team;
Inside teh trigger you can access and modify only the triggered record and only using :NEW and :OLD references.
Here's how to update a field on the triggered record of the table:
CREATE OR REPLACE TRIGGER sum_points BEFORE INSERT OR UPDATE ON team FOR EACH ROW BEGIN :NEW.Points := :NEW.Wins * 3 + :NEW.Draws; END;