The internet is by far the biggest market to advertise and sell in
, yet it is also the one that has a massive void that needs to be filled. I know that with pop ups and ads, it might seem like there might be too many ads around, but the fact is that there is still a lot of room available for people and relatively easy to get involved. Of course, it isn't as simple as putting up a website and slapping some ads on it to make money. If it was that simple everyone would be a millionaire. But if you're willing to put in the right kind of work you can make a lot of dough.
A big part of getting started is knowing what market you should be going in. It's no different than opening a store in your local town. You're not going to make any money unless people want to actually buy what you're selling. This is a relatively easy process with Google's Adwords Keyword Tool, which allows you to search for keywords inside any niche. It will tell you the average bidding prices on various keywords, which gives you a good idea of what you could make if you went in that niche.
You can figure out how much you would make by looking at how much traffic you would expect to get and what you'd probably get paid for a keyword. Typically you'll make about one half or one quarter of the bid price for a keyword. So if the price was $1.00, you'd probably get paid between $0.25-$0.50 per click. If you had 100 visitors a day with a 10% click ratio, you'd make $2.50-$5.00 per day.
I know making a few dollars a day doesn't seem like much, but imagine getting one site that earned that. You just leave it and move onto the next one. As you keep adding sites you'll make more and more money.