There are four main ways to make money online. Interestingly enough, there are also four main ways to make money in general.
You can sell advertising, information, products or services.
Online, you can run "traditional" banner or text ads or you can sign up for something like AdSense and run PPC ads. In the offline world, revenue from classified and display ads is actually how most newspapers and magazines make most of their money.
The secondary source of income for most newspapers and magazines is subscription revenue and per-copy sales. Basically, they provide useful information, which people are willing to pay money for, either by subscription (which adds the convenience of home delivery) or by paying per-copy at the newsstand. People buy books (and visit bookstores) because they want the information/entertainment the books offer.
Online, you can do the same thing by making your site a tremendously useful resource with information that people are willing to pay money for, and selling subscriptions that allow them to access the bulk of your content. Or you can write e-books and sell those.
Make original crafts. Sell handmade furniture, new & used office equipment, custom-designed jewelry, designer clothing, all-natural cosmetics, home electronics, sports gear, personalized greeting cards, organic gourmet food products... you name it. If you can think of it, it's likely there's somebody selling it successfully online. And if they can do it, so can you if you put your mind to it. So pick a product you like, and go to work.
Can you design web pages? Are you a marketing whiz? Can you help kids with their math homework? Can you respond to customer inquiry e-mails on behalf of busy business owners? How about a birthday/anniversary reminder service for forgetful people? "Hotel-style wake up calls at home"? There are all kinds of services you can offer. Let your imagination run free for awhile and I'm sure you'll think of plenty.
The point is, there are zillions of ways to make money online, just as there are zillions of ways to make money offline. The thing is, almost none of them will make you a millionaire overnight, and almost all of them will require work. Often a LOT of work.
Find something you're interested in. Something that gives you a sense of satisfaction when you work on it. Something that gets you excited. Then see what ways you can brainstorm to monetize that area of interest.
It's much easier to put in the time and effort required if you're working on something you enjoy in the first place.