Now what you are going to need to develop this are the following:
1) paper and pencil - This is where you create the game on paper and think up how everything will interact with one another. For example in your mafia game (btw you are not the pope are you?) you will want to write out a story line, possible venue locations, unit types (thugs, dealers etc), how you simulate a battle, how scores are done, how do you advance in the game etc. Once you got all that written out, you can move onto the next thing.
2) You are going to need a web host that can supply you with MySQL databases, has PHP installed, and possibly has the ability to run cron jobs (schedule programs to run at certain times to update the environment for your game)
3) List out the pages you need based on what you wrote on the paper, put some flow charts together for your code. Maybe even write out your pseudocode for various events you have planned.
4) Read up and study the PHP language. Then you can start with simple things like the user login script and how to connect to your database (google for articles on the topics. Some books also cover these topics.)
5) Write and learn as you write. Work hard and the rest will be glorious!
Hope these tips help you with the development of your game.
And if you are the pope, let some admin know that you are the pope and want to start a mafia game (its an inside joke).