using ID as a key

Hi,

I'm new to data base design so please bare with me if my
question is so basic..
I'm designing a database, two of the tables are EMPLOYEES and
AGENCIES. I need an ID for each of them. I would also need to
have the agencyID in the EMPLOYEES table so that I can identify
which agency they belong to something like a foreign key. I know
postgreSQL does not support it so how do I implement this ?
What kind of data type should I use for the ID ? In general I
do not know how to implement IDs and use them as keys.
I've read the user manual and a bit of the programmer's
manual several times but haven't found anything that will
apply to this situation.

Please Help. Thank You.

-Sheila