There are many different ways to make money online. You can start by building your own website and then selling advertising space on it, or you can sell other peoples products and services in an affiliate program. You can also do business with others on a direct sales basis.
However, if you are looking for something that has been proven to work over and over again, I would suggest starting a blog. The reason being is that it takes very little time to set up and once you have your blog set up, you can start earning money almost immediately. The best part about a blog is that you can use it as a marketing tool. Once people see your blog, they will want to visit it. They will then be able to click on links to your affiliate programs and purchase items from you.
I recommend using a blog service such as Blogger.com or WordPress.com. These services allow you to set up a free blog within minutes. Once you have your blog up and running, you will need to learn how to drive traffic to it. There are many ways to do this, but one of the easiest is to join some forums and participate in the discussions. When you post an answer to a question, make sure that you include a link to your blog. This way, you will be giving your readers a chance to visit your site and learn more about what you have to offer.
Once you have started driving traffic to your blog, you will be able to earn money by setting up ads on your blog. You can get these ads from Google AdSense or Yahoo Publisher Network. Both of these companies pay you when someone clicks on an ad on your blog. Once you have these ads set up, you will want to make sure that you update your blog frequently so that you are able to generate new traffic to your site.