how can i write a trigger that compare between two tables in phpmyadmin mysql
Here's one example. You should make deptor and mypin 
columns not null, or you will have to deal with possible nulls there:

delimiter //
create trigger before_insert_users
before insert on users
for each row
begin
    IF ( EXISTS (
            SELECT 1 FROM debtors
            WHERE debtors.myPIN = NEW.myPIN
         )
    )    
    THEN
        SET NEW.debtor = 1;
    ELSE
        SET NEW.debtor = 0;
    END IF;
end //


Note that you will need a delete trigger on debtor as well (if myPIN can change you need an update trigger in each table as well.