Direction Designing App with User Specific Data

[chopper]

I have built a internal shipping manger php/mysql application for a local transportation company that is working well. This has been my first journey into programming.

Now the owner of the company wants me to tweak this so customers can login in and enter their shipments online. I will be able to reuse most of the scripts I have already developed.

My question is this:

If I want a user to login and be able to use their own personal address book, shipment history, etc. can someone help direct me on how I should go about doing this.

The script I have now is single user only so I have not had to tackle this.

If I am going to switch it to  a multiple user application where each user requires their own private area, do I create a table for each user?I know there are plenty of different ways to do authentication via PHP/Mysql but once the user logs in how do I limit them to their own data?

Any links or ideas where to start designing this would be much appreciated.