Using indexes

Hi,

Indexes are used to find a record faster in a table.

It only seems to work if I am working with one table.
eg)
> create index customer_id_idx on customer(id); 
> select * from customer where id=1;
-- This uses the index I create

However, when I start to join 2 or more tables together it doesn't seem
to use it.. WHY???

eg)
> create index customer_id_idx on customer(id);
> select * from customer, purchaseorder where customer.id=purchaseorder.id;

-- This results in a seq scan .. WHY??

Is there a way to make it so that it uses my index

Thanks again.