I receive more emails from people asking my opinion on their website/blog idea, than about anything else.
This article should help you decide whether your idea is feasible or not.
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. Even if your site doesn't succeed, these small ventures very often lead to new and bigger things that do succeed.
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, you'll likely get stuck or bored very quickly, and give up. (In case you're curious, by far the most profitable topics are those that are money-related. Insurance, loans, and mortgages.) If you're not sure what you want to write about, choose the topic that interests you most.
You should set out to make your site the best site in your field. You do this by choosing a very narrow topic, in which you can actually beat out 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. Later on, she branches out into a few other flowers.
People with no internet experience often contact me to tell me about their grand ideas for major new websites. They have no idea of the economics of the internet, or what to expect from a new internet business. Most of these grand ideas require large, custom-built sites, that would have to be built by a professional, and cost a five-figure amount of money.
I've found that many people with website business ideas also have some funny ideas about getting out of paying for anything, through some kind of favourable "partnership" scheme. I've received many proposals over the years from people telling me "you create my fantastic website idea for me for free, and when I'm a hit, I'll give you 10% of the profits."
If you believe you have a great idea, you need to earn and risk your own money, and reap your own success or failure.
To try something big like this, and succeed, you need serious internet business experience, or serious real-life business experience in the field the website deals with. Leaping into a large-scale internet venture, from nowhere, is a sure-fire recipe for failure.
You can make it big online – I've seen people do it – but you need to start small, and learn the ropes. If your site turns into a big, successful site, it will have started out as a small, simple one. If it doesn't work as a small idea, don't do it.
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.
A classifieds site, social network, or auction site all require a critical mass of pre-existing users, to be successful. If you try to create such a site, it will almost certainly fail. No one will sign up for an auction site where there aren't already people using the site.
"Network effect" sites either start off by providing a high-quality service in a small niche, where they can survive on a very small membership base, or they're suddenly created, with a huge burst of advertising, by people with serious experience and resources.
Social network sites also have the problem that they require a reasonable amount of initial setup. The user has to learn the interface, add friends, and join groups, before they can start to benefit. Most people aren't interested in getting involved in a new and tiny social network, when they're already on Facebook. Consider whether you actually need to create a social network. Often, a discussion forum or blog is far more appropriate.
Examples of sites you could easily create: A business site with contact details, a photography blog, a church website, a family page with photos and a blog, a site to sell downloadable audio files, an "ask me a question" advice blog.
A WordPress blog/site can contain any amount of pictures, text, pages, and links. If the site you want to make is just pages of text, photos, videos and/or links, then it's dead simple. You can also sell things via PayPal or credit card. 90% of sites should fall into this category.
Sites with more complex functionality can be created, but they require the use of plugins. These are easily-installable extras you can add from inside WordPress. If what you want to do is fairly normal, there should be plenty of plugins that can do it. You probably won't need to deal with anything highly technical, but you will likely need to do your own research and experimentation. If you're technically-inclined, and happy to play around with things, then this is something you may be comfortable with.
Examples of sites that would fit this category: A site with a complex shopping cart that calculates shipping costs by weight, a site where people can have their own pages and upload pictures to them.
Complicated, unique, interactive websites. Any site that stores or calculates information, and isn't something normal that a plugin would exist for. Any site that has a very specific layout requirement.
This kind of site is well beyond the scope of a novice. You'll probably need to hire a professional firm to program such a site.
Examples of websites you probably can't create yourself: A site that calculates a travel itinerary and costs, a site where people can upload/listen to/rate music files, a site where people can search for nearby businesses, a site where people can host games they've made, a site where joggers can create a profile and input their jogging statistics.