First what interests you? You may not feel you have a wide body of knowledge for one particular think but there has to be something that interests you in life. Hobby? Sport? Start there.
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Don't put the cart before the horse. Plan your website, thencreate it.
Lisa's advice is right on. No one can suggest a subject for you. You need to choose a field that can really hold your interest long term because it will take a few years to really build a solid business.
If you get passionate about any subject you can learn quickly what you need to know to get started. If you start reading an hour a day you will read a book a week. Once you have read ten books (paying attention) on any subject you will rarely find another person who knows as much as you do about that subject.
Don't be intimidated by what you think others know. No matter who they are they once knew nothing about their field. If they learned their topic, you can learn yours.
Just remember Lisa's advice. Write within you knowledge as you go. Don't try to sell what you haven't done. But on any subject you can start with "The basics of..."
Yeah, you need to focus on something you enjoy doing and you will enjoy talking and writing about as its proven if you don't enjoy doing it. It wont last.
Narrow down your niche as far as you think it should go and start reading up on it, research, brainstorming and gathering ideas. This is the most important step when creating your site so don;'t rush it to much.
Oh man is this true! Years ago I rushed into building a site about Genealogy. I knew a little about the subject, but not enough to create the quality content needed. I realized my error when guest would ask mean questions that I had no clue how to answer. It was a real wake up call. I really did not have the passion for the topic that I needed although I was getting great traffic to the site after just a few months.
Go with a topic you really enjoy talking and writing about or you will burn out quickly and give up on web design.
Good Luck!
__________________ GrafxStar "Growth begins when we begin to accept our own weakness." - Jean Vanier