The first thing I wanted to know after learning the basics of PHP was how to store my data on a more permanent basis. PHP is all well and good but without the ability to store data between browser sessions it has limited functionality. After learning how to store and retrieve data, your PHP scripts really come to life.
One of the biggest mistakes of today's fledgling MySQL programmers is their field type definitions, also known as data type or column type. Many are too quick to set up their tables, flying through the different options without giving it much thought. The time you spend setting up your tables will save you from many headaches later on down the road.