www.mysql.com should be where you should be looking for all materials. Reading about, working with and trying out different things is what would be the best possible approach for gathering knowledge.
So download the MySQL Server, install it (following the guidelines mentioned in the reference manual) and start working out different things with it. MySQL Server's reference manual is considered to be the best source of information about it. It is more than 2000 pages long, with detailed information about the internal working of all things. You can download the manual from the site's documentation pages. Working with the reference manual you can try out the different configurations of the server and get to know which is best suited for what type of use, you can learn about all the data types and how to use the disk space judiciously by assigning the best suited data type for a parameter, you can find out the different storage engines or tables types and what typr of locking mechanism do they use, figure out which storage engine is capable of doing what and hence when to use which, the manual also lists about optimizations that could be done so that your database achieves best performace and a lot of other things that informs you about the best practices of operating a MySQL Server.
You could also look at the Certifications offered by MySQL, these are a way of assurance of your knowlegde to a prospective employer. So you could prepare for and appear for certifications exams. Information about certifications is put on the site too, they also recommend books for certifications and for general reading too.
Lastly you need to be aware that gaining detailed knowledge and masterful insights about all modules of such a huge project is never going to be an overnight activity, so you need to have patience and perseverance and all along you would need to be constantly working with the server this would help in your overall familiarity about the server.
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