I've been asked this question countless times. It's rarely a technical question. Usually the person is just seeking reassurance that what they're doing absolutely makes sense, before they go ahead and do it.
If you're one of these people, this article should tell you what you need to know.
I respond personally to almost all legible emails I receive. Unless there's some obvious problem with someone's site or blog idea, I've always said "yes, that sounds like a good idea for a blog (or website)".
Fundamentally, the answer is that if anyone would possibly be interested in what you have to say (or you simply just want to get something off your chest), then it's a good topic for a website or blog.
Many people write about their personal stories – recovery and rehabilitation, travel, family, war, or just their life in general. Others write about their own personal opinions – world affairs, philosophy and politics. For some, their site is an outlet for their creative work – poetry, musings, creative writing and stories. Whatever you want to write about, chances are it's pretty normal.
To make a great site, you need to make a site that deals with a topic you really understand, or an idea you're passionate about. If you just pick a topic because you think it will be profitable or fashionable, you'll likely get stuck or bored very quickly, and give up. If you're not sure what you want to write about, choose the topic that most interests you.
If you want your site to be a big success, and grow in readership, you should aim to make your site the best site in your field. You do this by choosing a single, very narrow topic, in which you can outshine everyone else.
Jennifer knows a fair bit about flowers. She could make another average site about flowers, but instead she narrows her scope, and focuses on making the best site on the Internet about her favourite flowers – lilies. She keeps her backyard bird-watching information off the site – maybe creating another less-serious site or blog about it. Her site eventually becomes the authority on lilies.
I don't recommend trying to make multiple sites. You can do it if you like (each extra site requires an extra domain name, but it can be hosted on the same web hosting account), but I strongly recommend you start with just one site. That's enough of a challenge for a beginner.
Setting up a website or blog is very easy, and the cost is trivial. Even if you're not sure of your idea, you have very little to lose by trying it, and a lot to gain if it succeeds.