You have so many errors I wont attempt to fix it. You can't select from a table that you have the trigger on. This will cause a mutating table error. The declare statement is not used in a trigger statement. For counter, which I assume is the primary key, use a sequence then your trigger would look like.
CREATE OR REPLACE TRIGGER GAL_TRIGGER
BEFORE INSERT ON USERS
FOR EACH ROW
You do not need a parachute to skydive. You only need a parachute to skydive twice.