I suggest that you start to develop small things and then bigger and bigger. You could for example start with a calculator (does not require database access) if you know enough PHP already, then go on and create a login script that uses the database. You can use the login script later in the auction site. Then do a few other small things, like a register for your music/books/anything. When you have figured out how to do that I think you are ready to begin with the auction site.
Of course you should read a lot and ask questions when you need it. Search for things you don't know about for sure, and use the information. A few things you should search for is "database normalization", "database injection", "security", "session security" and "login security". Be sure to program it totally secure, if there is a way to "hack" that kind of site it will be "hacked".
Another tip is to design everything (database, webpages, scripts) before you start to program. Otherwise you will only be doing everything 3-4 times.
Good luck with the project m8 😃
