MySQL joining tables...

Hello, I am pretty new at this stuff but here it goes,

I have 2 tables, one that has
$query="CREATE TABLE products (id int(6) NOT NULL auto_increment,fname varchar(15) NOT NULL,lname varchar(15) NOT NULL,stage2 varchar(20) NOT NULL,itemnumber varchar(20) NOT NULL,quanity varchar(20) NOT NULL,email varchar(30) NOT NULL,location varchar(30) NOT NULL,PRIMARY KEY (id),UNIQUE id (id),KEY id_2 (id))";

In the 2nd table I want to have 3 items, itemnumber, quanity, and descrition.
I want itemnumber from the 1st table to refer to the 2nd table and update the quanity. For example, If I order 3 of item #12345 then i want it to update the #12345 to 3. And then say in a week someone else orders 4 of #12345 then i want it to change to 7. How would I go about linking these tables together. Any help would be nice. Thanks in advance,

Justin