Have you settled on a templating engine and a db abstraction engine ? Those two things would definately help you get started. My choices would be Smarty and ADOdb.
Once that is done, then I would work on the database schema so that it could hold all of the data that I had for it. Then, it is designing the screens (login, latest news, main index, main admin, wrong password, email password, register, individual post, extended post, etc).
Once you get to this point, 80% of the work has been done, you simply code the DB queries, load the templates, replace variables in the templates, and output the pages - for all the actions that you want to occur.
That's probably backwards compared to what some people would tell you - just get in there and start coding... - but it prevents you from being suprised - you want it to store THAT in the DB ? - and in my experience, results in a completed application much quicker.