Before beginning to write anything, before setting finger to keyboard, before drawing any images, before thinking up a logo, before registering a domain name, and before anything else, ask yourself
- What the heck do I want to say?
- What is this about?
If you head to that street corner and begin rambling on about a thousand things all at once, will anybody pay attention? Probably not.
Knowing what your message is will let you focus on that message, and should reflect in a successful web site.
Having said that, it's not necessary to go overboard with developing your purpose for writing a web site. Let the purpose grow on you, because it will anyway. Do the best you can right now, and expect the purpose to grow and evolve over time.