PHP/ASP are serverside meaning, the server does all the equations and returns it as HTML. This allows for things like forum's, counters, polls, and etc..
I meant to add ASP tp the list, and I did mispell it ( ASL lol ). But you recommend PHP... does every computer support it?
PHP can be ran on Linux and Window servers. ASP is limited to Windows only servers. PHP can be set up on basically any computer via Apache. Asp isn't free and PHP is. PHP is also more commonly used. I heavily suggest learning it. You will need Windows Xp Pro in order to set up ASP on your computer, with PHP you can easily do it via Apache(for really quick debugging).
Now where to start?
Get your game down, get the basics. Once you have that, start outlining how your database is going to be structured.
Do some other projects to. Like a project that deals with login/logout and creating of accounts. Do a bunch of smaller projects that will give you experience for the big project. That way when your half-way through the big project, you wont realize you missed something BIG and now you have to rewrite everything. Look at sites that talk about security and what to use and what not to use.