newbie db design question

create table products (

product_id serial primary key,

description text,

supplier_id????) ;

create table supplier (

supplier_id serial primary key,

description text) ;

the products table should be linked to the supplier table via "supplier_id" column. i can't find out what would be the data type of supplier_id in table products to do that. i tried

supplier_id serial primary key references supplier

but it won't allow multiple primary key.

how then to proceed? 

regards,