Writers have an advantage over other folks seeking to establish a presence on the internet: content isn't a problem for us like it is for many individuals. Even better, if it's your own site, you get to set the word limits!
A writer's website is an elaborate portfolio, showing how you write on each page. Don't overly sanitize your writing. Let your personality bleed through. Your website needs to be about you, but designed to meet the needs of people looking to locate and hire you. So, start with the basics: who you are. Let them see you with a picture and examples of your writing that capture your unique author's voice. What will an agent be looking for when she goes to your site? Proof that you can write and produce the goods, proof that you have something worth selling. This agent will also like to see that you are capable of marketing yourself, and having a ready-made following is a selling point.
A following? You know -- your fans. Of course you have fans. They may be limited to friends and family right now, but that's a start. It can be hard to think of marketing yourself in such a blatant way, I know. I resisted it for years, even while learning about marketing in other areas. The thought of marketing myself and something as personal as my writing...well, it still gives me the shivers. But you can do it, and you may find -- like I have -- that you'll enjoy it.
These agents, publishers, fans, will all need a way to contact you. You choose the level of contact you want to have. I recommend starting with some sort of filtered e-mail, or a contact form if your server allows it, preferrably with one of those little "captcha" spam-prevention boxes where a real-life human being has to read a picture and type in a word or phrase. Otherwise, you won't see the important emails sent by potential agents or clients, simply because you'll be digging through 3,895 ads for a variety of drugs.
If you're published or if you do speaking engagements, you can include information that the press can use when they want to write about you. Other items to include can be testimonials from fans, links to buy your books or other memorabilia.
Writer's Digest Magazine has published a wonderful article on what a writer needs in a website in their October 2008 issue.