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