Neo,
Keep a small pen, and a piece of paper or a notepad with you at all times.
As you are going about your daily business, or surfing the web, keep yourself open to any ideas that pop into your head.
Write down every idea you have, no matter how silly it might seem. I have a list of 100 website ideas, some good, some ridiculous, some might just be brilliant. (my favourite ideas will be coming online in the next few weeks stay tuned!) But the important part is to keep your creative juices flowing and accepting every idea you get without judging it.
Once you build a habit of making up new ideas, and accepting them, you will start to get your good ones coming to you.
Keep in mind that your website doesn't have to be about -your- interestests, you can get someone else to provide content while you provide the site, or the idea, or even just the kick in the butt to get them moving and writing
Then you agree on a profit share.
I am planning down the track to have some friends of mine produce blogs, e-books etc. on topics I know they are really good with, and I am going to simply build the site for them and do the marketing, and we split the money.
You have a LOT more options then, and you can often build it and walk away while they plod along provide content for you!
As someone said above, study internet marketing. I am cramming myself through a number courses while building sites now, and having a great time. A lot of stuff I didn't think about that I am sure will make a huge difference in the long run.
Most of all have fun and don't stress! If you are worrying about what type of site to build, the stress will prevent you from thinking clearly, and stop the creative juices from flowing.