going crazy with serial type

OK.  My background is in mysql, and I've been converting over to psql.  Just
by way of background.

I do !NOT! understand how the SERIAL type works.  I want something
like I had in mysql that would generate new, unique numbers, each time
I added a new record.  I want something that sits down, shuts up, and
just GIVES me the number on demand.  (I also want a guarantee that the
unique number is consecutive, and is never zero or negative.)  In short,
I want the AUTO_INCREMENT behavior.

But so far it's been one giant headache.  Tell me, how do I insert
new records into a table *without* specifying an actual number?  In
mysql it's just an empty field.  I have been unable to determine how
to do this in psql other than to ascertain it certainly isn't through
the same way.

I've been through the documentation, but for some reason, no one seems
to think a programmer would ever want functionality like mysql's
AUTO_INCREMENT, so as far as I can tell, it's not even addressed.

I'd appreciate any help.  I basically have a table:

create table mytable (mytable_id serial, a int, b int);

and

insert into mytable ('', 1, 2); is accepted but then following
insert into mytable ('', 5, 6); etc, is rejected due to "duplicate key"


Thanks,
--Cindy
--
ctmoore@uci.edu