Foreign Key: what value?

Hi,
How to know the value which I must set in the foreign key field?. I have two 
tables:


CREATE TABLE AAAAAA ( Id  SERIAL PRIMARY KEY, data char(9)
);

CREATE TABLE BBBBBB ( BBBBBB_Id integer REFERENCES AAAAAA(Id) NOT NULL, field char(5)
);




I insert a register on table AAAAAA,
  INSERT INTO AAAAAA (data) VALUES ('123456789');


and then, I want to insert a related register in table BBBBBB, but I do not
know how get the proper value to the Foreign key BBBBBB_Id. Note that a lot of 
client are inserting at the same time on the AAAAAA table, so I can not just 
get the greater value of AAAAAA.Id

Maybe using transactions ?. Any tip, URI, ... will be welcome.
  INSERT INTO BBBBBB (BBBBBB_Id, field) VALUES (??????,'12345');


Regards,
Davi