Where to start!

First I would like to say that I have had PHP experience but not enough to say I am a PHP developer. I want to use PHP to build a site that uses MySQL, I am building it from scratch so I don't know where to start, should I start with PHP and design a database around my code, or should I designs the db and design the site are the db. is it just a matter of preference or is there a recommended way to do this? Thanks