Row ID and auto-increment?

If I create a table like

create table tablename (
aNum integer not null,
name varchar(10)
);

If I do select * from tablename;

q1. Is there such thing rowid similar to Oracle in PostgreSQL?
q2. How do I make aNum auto increment by 1? Need to write
a trigger? how to write that?
I want to enforce column aNum 0,1,2,.....n.
I want to prevent data entry people input 0,1,4,5,8,...n.
Thank you very much in advance!

--Raymond