using IDs

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 manual many times
but haven't found anything that will apply to this situation.

Please Help. Thank You.

-Sheila