I appreciate your desire to learn, but I think you must really start with some reading and general learning about databases, before you can even ask focused questions. General questions are impossible to answer without a lot of back-and-forth dialog to understand what you know, what your logical requirements are, and where to even start giving you information.
Try reading the MySQL and PHP tutorials at www.devshed.com and you will have a start to understanding this. Also, check out the following resources:
In other words, you need to start by building some generalized knowledge of databases before you can ask intelligent questions. And, the best questions happen after you have tried something, not before. Try starting a database, even something really simple, and if/when things don't work according to plan, then come back, post your design here, and you will find some help .