How does one get started on creating a social community web application?

[eluser]shibuya[/eluser]
How does one get started on creating a web application for social network? I want to create a web application that allows registration/registered members to share information with each other. Things like ability to make comments on a blog post, send private messages, track profiles, vote on posts, make blog posts, have image avatars, upload images etc.

Not a huge web application but just something simple but I want to code it from scratch, without using any cms or bloated free software online.

My background is moderate understanding of PHP (procedural programming, still learning OOP/MVC) and very experienced with XHTML/CSS. I always used Wordpress or ExpressionEngine to create my sites prior but they don't really give me the function or satisfaction of creating a online web application with features mentioned previously.

Thanks.