What will your site be about?
If you can't answer this question already then you've got a long way to go before you are ready to start putting things together and I'd suggest that you have a serious rethink before you actually start building the site. Of course, if you are doing a business or company web site you will have the content of the site decided for you; making the decision about what to put on a personal or hobby site is not so easy.
What interests you?
If you are making a personal site, then by far the easiest thing you will find to write about is something you are really enthusiastic about. Typically this will be about a hobby that you do in your spare time or perhaps about your favorite movie star or movie. Whatever you are going to base your site around, you will find it so much easier to write about something you are interested in rather than something you couldn't care less about.
Don't worry if your interest is rather unusual in the area where you live, the Internet is a large place and there will probably be more people interested in something as obscure as collecting bottle tops as you might think! Infact, if you have a rather unusual hobby and you know that there aren't that many good sites for it to be found on the web, then this could prove to be a good topic for a site as you could easily fill that niche. Who knows, it may even become profitable if you can find the right sponsors for it!
How much do you know?
Don't feel you have to limit your site to be about the facts that you already know - turn your site into a learning experience and research your topic to fill any gaps you feel you have. This is one of the reasons why I feel it is important to write about your interests; you will find such research more enjoyable and rewarding. If you've got plenty of books or magazines on your topic, look to them for inspiration as to the content you might want to expand on (don't however just use any articles without permission). Don't feel that you've got no idea on where to look for more information either, you don't really need books from the library to find what you are looking for - you have a large World Wide Web full of information out there, make the most of it!
Keeping your visitors interest
Its one thing to get your visitors to visit, but if you've got nothing new worth seeing the next time they visit then they may never walk through your corner of cyber space again. This is where your knowledge as a visitor to similar sites comes in handy. What would make you go back to a site? Is it the best place to get the latest news on your topic or does it have a nice little community of people that use the message board? Whatever it is that keeps you coming back, you should try to incorporate that idea into your site - however, a quick word of warning... do not try to replace something that already exists because it is likely to fail (for example a top sites list when there are already too many out there for your interest).
When I first started out on the web, I built my site to show off my Petz. Seeing how I didn't think that many people were interested in them, I didn't expect many visitors; nor did I get many. However, what I did get was a steady stream of regular visitors because there weren't many Petz sites out there. When I started to promote other sites via the banner exchange and top sites list that I ran; I got many more new visitors because the regulars were linking to my site in many different ways. The site in question is now well known about to those that use the Petz program.
The most successful site ideas have come from those that fill a niche in the online world; if you see a gap you think you can fill then go for it!